It took me 7 days primarily low betting the whole time to reach level 100. (9 to reach level 110.) Whenever I bet higher than 350-450K, usually, my coins just plummeted within minutes. I think anyone who reached level 100 within 1-3 days bet higher and lucked out tremendously.
Slow and steady guide:

1. Join a club. The higher AND more active the better. Absolute highest is Master 1. From what I've seen, Master I, II and III required players to be a minimum level (like 100+) to join and/or a request to join (mine was never accepted). Unlike similar casino games, Stars Slots won't let you do an advanced search. You can search for a specific club if you know any names, however. So you are left with the option to join the first club on the list that currently has the most collective coins made and which allows members of your current level to join. Either that or create a club (don't recommend). So go ahead and join that first one, but change later based on step #3’s information. Also, the higher the club the bigger percentage of extra free coins you get from different sources within the game. So it goes Master I, II, III, World Class I, II, III, Expert I, II, III, and last is Pro I. Since I couldn't join Master clubs I did best in World II and World I.
2. Collect coins. The FB group, free coins from https://www.peoplesgamezgiftexchange.com/stars-slots-free-chips/gifts or similar website, starting with the most recent and going back as far as you can. Within the game: Once daily spin the wheel; Star Bonus every 60 minutes from far bottom left side of home screen; Huuuge Heist (unlock at level 35) every 15 hours (beside Star Bonus) (the 15 hours increase the higher a level you are at for longer wait)... make sure you've gathered bucks (can see next to dollar image under coin image at top left) (amount of bucks needed increase the higher a level you are at) in case you get an empty box and want to keep what you've won so far with boxes. Tip: An empty box for one round, for example the first one, can also be the empty one in the very next round; it's random each round. I was terrible at this game and should've clicked "give up" sooner than keep gambling; Lottery (ticket to play) every 6 hours (some say value goes up at level 70, so to save them. User batmanbnb below states to save bronze tickets until level 70. It’s up to you whether to save tickets or not.); click on Shop (top middle in green) and on the top right you can click and collect free coins every 8 hours; at the bottom of the Shop screen OR to the right of the Shop image on the home screen you click and watch a video to collect coins, until you can no longer watch, every 8 hours. Also collect coins from club members every time they hit a jackpot and 1 million coins just for first time joining any club through Stars League (middle bottom of home screen).
3. When you join a club click on a team member's profile and connect and you'll get an option to connect/friend every member in the club. I did this and connected to as many people during games I played until I reached my limit. I figured people in the clubs I joined, even lower level clubs, may leave and join higher ones like I did. Probably would be a good idea to go to a tier II game (the middle option for higher betting than Beginner called High Limit once you've unlocked it) and connect to players currently playing it since they have high enough coins to play that. You don't have to play yourself if you don't want to: just drop in, connect, and leave. Then, at the bottom right of the home screen click on Social and you'll see every friend you have. What I did was click on those who had the highest coins at the time and see what club they were a part of: If the club was World II or higher I joined it. Question: What if you want to do better than this club you found (either through this method or the Join Club list) either because its level is too low or not active enough players? Click on Stars League (middle bottom of home screen) then click on Seasons (to the right of Members). This shows you where your club is currently placed. If you see higher placed clubs with available space AND you can join AND it's World Class II or higher then join it. Remember your old club’s name if you ever want to go back, just in case. If you attempt to join a club and you can't get in (maybe you're not at high enough a level) you are also kicked out of your current club for attempting to leave (for some reason), leaving you club-less. My favorite clubs were Polacy2019 (currently World Class I), pour tous (World Class I), and A.Y.M.K.N. TEAM (World Class I).
4. Betting: The general rule of thumb I found was to bet when you have a minimum of 10-20 million, which I agree with. But you can wait, collecting as many coins as you can, to start betting when you have more. The tip I read (and stuck with) was to follow the 1% bet rule: bet 1% of your current chips or even lower (like half of that).
5. Games: Everyone has a different favorite game. Some games are passive (can do Hold for Autospin and not babysit the game) while others you need to be engaged with*. Passive games: God of Sky, Steam City, Neptune's Mistress. *Note: User DilutedDante below stated all games are passive; for the games I saw that wanted my input (choose numbered cards for spin the wheel, etc.) I didn't wait long enough to see if the game would perform the action for me. Recommended games from other users here on Reddit: Wheel of Wins, God of Sky (one of my favorites), Speedy 777 Spins, Retro 81 (one of my favorites), Fortunes Jackpots (never played), and Jackpot Mania (never played). Of course, more games are unlocked the higher the level you reach. I recommend that if you are under 30 million to play God of Sky or Speedy 777 (which didn’t work too well for me, but others like it when trying to gain coins). Remember to do minimum betting or 1% or lower betting. Occasionally I would switch games and play Retro 81 or Wheel of Wins. Do this until you reach level 38 and unlocked Steam City and level 40 to unlock Neptune’s Mistress. I almost exclusively played these two, passively, to reach level 100. My method was betting 100K or 125K (if I had a minimum of 40-50 million). I didn’t feel secure enough to bet 250K or higher until I had a minimum of 100 million because otherwise I would run out of coins too quickly. I rarely bet higher than 500K. I think it took 2 hours or so, minimum betting, to level up each time. So that’s why it took me a week to do this and reach level 100. I found Steam City to be the best game overall as another user did. Sometimes, even at lowest betting and even with 100+ million coins and leaving it to play on its own, I would lose all coins. But that’s OK - just collect coins again and play again. It’s all luck. Another time I started with 15 million and it jumped to 125 or so million in a short amount of time with 100K betting playing Neptune’s Mistress.
6. I don’t recommend Daisy’s Challenges (far bottom right of main screen). The games to be played either rarely had wins for me or the minimum bet required to unlock a box was higher than I was comfortable doing. See user DilutedDante's comment below for benefit of doing Daisy's Challenges.

If anyone has any other tips or corrections on what I've said (I've read other users got into Master clubs, but they all seemed to be for very high level players?) please share! This is just my conservative and mostly passive playing guide based on how the experience was for me. Some users do recommend Daisy's Challenges or betting higher playing the 2nd tiemiddle High Limit slot machine and had better luck with those than I did.
I gathered many other tips from various users from the Swagbucks Reddit thread not mentioned here. If anyone wants me to include some of them I can.
See below for user Kpheark1's additional tips.

This might not sound important but the offer is easily completed in 4-5 hours and u/Mikazah made a wonderful guide on how to complete it. one tip I would give it to do the Huuuge Diamonds casino. It doesn't have any free spins but every so often it hands out a chest with chips in it and the higher your level your jackpots range from 100m chips to 6B. Same thing with the chests.
Edit: Alot of the comments said that it would take more than 4-5 hours depending on how lucky you are.

Overview

Edit: Just to make things clear: There's an offer on AdGate to reach a certain level in this game. Your earnings aren't determined by your luck with gambling.
 
As the name implies, this is a slots game. That means it is entirely luck based, and every user is going to have a different experience with it. I ran out of chips at level 53, the next day I struggled to gain chips, and the following day I woke up to 450m chips which I then quickly tripped that to 1.2B chips. This barely got me to level 100 before suddenly winning another 3B chips on my last auto-spin.
This game frequently gives out chips, and there isn't a specified deadline. Considering this game practically plays itself, it's definitely worth doing.
Edit 2: I took a look at Huuuge Casino Slots as mentioned by u/jraspberry and it seems to be almost exactly the same. There are just a couple differences with names. This offer is $17.66 with 2x Adgate on Swagbucks.
 
^{Just in case someone doesn't know the common conversions - K stands for thousand. M stands for million. B stands for billion. So, 500m would be 500 million or 500,000,000.}
 

Offer Name	Billionaire Casino™ Free Slots 777 & Slot Machines
Offer Wall	AdGate Media
Device Used	I use Android. I do not know if it works on iPhones.
Amount	I got $18.40 from Swagbucks on their 2x SB day. Today 2/7/20 is a 2x AdGate day.
Time Limit	None mentioned.
Time Spent	I got to level 53 the first day before running out of chips. I completed the game in under 3 days. Total hands on time was maybe an hour tops. It mostly plays itself.

 

How to play & Tips

As the name implies, this game is about slots. (Note: They do have other games, but I didn't touch those.) Thankfully, they also include an auto spin feature, so you don't have to give it much attention. Just long hold the spin button until it starts auto spinning, and away you go. You may need to play the first game a couple times before this unlocks.
The goal of the game is to get to level 100. You gain XP simply by playing the game and spending chips. Spending more chips will give you more exp, and it seems to go up at a linear rate. I chose to bet 1% or less of my balance to prevent any heavy losses because my luck generally sucks. While it does feel slower, the game is playing itself, and I wasn't in a rush.
As you level up, you will gain access to more boards. If you are having a bad streak with one game, you can just swap to a different one. Make sure to use up any free spins before swapping games though, as they are forfeited when you leave. There is also a "Basic Pass" event you can do and you'll get a good amount of chips for hitting the goals for certain games. You can find this on the home page or in the individual games. If you're going to play a game anyways, might as well get the bonus. (Note: I played for less than 3 days, so this event might change).
At certain levels, you will get a 2x XP bonus for 5 minutes after hitting that level. These are often in the reward box every 5-10 levels. However, it also seems to happen with every level up starting around level 80-85. Since it's timed, it may be worth upping your bet during this time period.
Every so often you'll get a mini game that pops up. Some will autocomplete, others give a higher bonus for doing yourself, and some just disappear. If you notice it, it might be worth doing manually.
You will definitely want to join a club. It gives you 1M chips just for joining, and you'll get all sorts of bonuses. You can do this by tapping the Billionaire League button on the bottom bar of the home page. There didn't seem to be any individual participation bonus for participating in the club events, so you can chose whether or not to do them.
 

Getting More Chips

There are a number of ways to get free chips in this game.

• You can get the Billy Bonus every 15 minutes. It is located on the bottom left hand corner of the home page. It doesn't give a lot, but it helps with the daily quest.
  
• You get one free spin of the lottery every 6 hours. You will also get tickets as you play, usually from leveling up. This is located on the bottom bar on the home page.
  
• You get a free Shop bonus every 8 hours. This is located on the bottom right of the shop page.
  
• You get a bonus for completing daily missions. You get 100k for each of the following: Spin 250 times. Hit a big win 3 times. Hit a free spins 3 times. Collect 3 Billy bonus. You also get an additional 500k for completing all of those.
  
• You can get club bonuses. After joining a club, click on the "Billionaire League" button located on the bottom bar of the home page. This will take you to your club page. From here, you want to click on the "Club Wall" Tab. You then need to tap each one of the green buttons showing you a chip amount. Make sure to check on this every 12 hours or less, as some of these will expire in 12 or 24 hours.
  
• At the start of each day, you will be given some chips. You get 1k chips per friend added (up to 25 friends). You will get 100 chips for every like you get (up to 100). You will get 1k chips per day streak. For example, on day 3 you will get 3k chips. You also get a spin of the roulette that will award up to 1.5M chips.
  
• You can get 500k chips for connecting to your Facebook account. This seemed like a lot at first since you only start off with 5M chips, but in the long run it didn't feel too necessary.
  

 

Should you spend real money?

Spending money would definitely speed up your completion time. It's by no means necessary, but even spending $1 or $2 would let you finish the game quickly. Just look for the popups where you earn 300-350% more. I frequently saw a $1 offer for 70M chips, and a $2 offer for 168M chips. You can pull these offers up by hitting the "News" button on the bottom bar of the home page.
That being said, considering the game practically plays itself and there's no time limit, I didn't feel a need to spend anything.
 

Other Features

This game has a number of additional features, which you don't necessarily need to participate in as they don't benefit you. I'm putting them here for completeness, but you can just skip over this section if you aren't interested.

• You can give drinks to users. I'm not entirely sure what this does, but I think it might send chips to others. You can buy the drinks for points. You can also give a free drink when a certain condition is met (it never stays up long enough for me to read it). When winning a jackpot, sometimes it'll let you send out a round of drinks to the other players. You only get 3 seconds to do this, so accept it quickly.
  
• You can like users every 60 minutes. This will help with their daily goal.
  
• You will get some diamonds. They seem to only be good for unlocking boards before getting to the required level and for giving out "drinks" to users.
  

 

General Tip:

When you click the offer on the offer wall, download the app right away. As soon as it's done downloading, open the app right away and let it run for a while (play the game, make an account, let it sit, whatever). Do not navigate away at any point during the process. Taking too long or messing around while the game is downloading/installing may cause problems with it crediting later.
 
 

Common Questions

Can I do this if I already completed it on another offer wall or played the game before on my own?

Generally, no. Almost all of these games state that it's for new users only.

What if I do it on another site?

No.

I don't see the offer! What do I do?

It's possible it simply isn't available for you. I'm from the USA, and many offers are not available outside the USA. If you are from the USA, try checking on the site I did the offer on or on a different site. Sometimes they're only on certain sites. It's also possible that the offer is no longer available.

I completed the task but I didn't get credit for it! What do I do?

First, wait 24 hours. Most of these games credit within a few minutes, but they may take up to a day sometimes.
Next, check to see if the offer has been completed through the offer wall. If it has not, you must contact the specific offerwall that you completed the task on. For example, if you do this task on Site-xyz through AdGate you have to contact AdGate. If the offer shows as completed but you did not get your points, then the points may have been held and you will need to contact the specific site.

This takes so long! Why would you waste your time for a couple bucks?!

I play these games in between other work or on the weekends. I don't do them instead of better paying work, but rather along side it in my down time or while I'm watching TV. Most of it is just waiting around for it to do its thing. I'm also a gamer, so I enjoy playing games.
 
 

Have any more tips? Let users know in the comments!

 
Looking for more ways to get paid to play games? Take a look here.
 
^{Is there a certain game you're interested in seeing a guide for? Let me know in the comments! Be sure to include the offer name, which offer wall it's on, what site you found it on, and approximately how many points it's worth.}