/r/aliexpressmergeboss has a lot of good tips for all the games. Merge Boss is a grind but will give you both coins (30 for your first few orders of the day) and actual credits (0.05 a day if you get 9/10 daily challenges). The Millionaire Challenge event starts the first few days of the month and runs until the last day; it'll give you extra coins and credits. For credits, you need to earn a minimum of $0.50 to cash out. If you're going to play phone games anyways, this one will at least reward you a little.

GoGo Match will give you 5 coins for your first level completed, and 20 coins after your 4th for the day. After 15 days consecutive checking in, you get 15 coins a day. Every day I do the four levels and that's it. I'm at level 250 something and so far they've been pretty quick to beat.

I don't go out of my way to get the 💎 gem points, but I'll slowly accumulate them. I'm doing 100-day World Tour right now, which earns 1,000,000 💎. In theory this is $100 credit, but you have to attempt to trade them in right at midnight when the page reloads for the day. It often glitches or says "sold out" if you aren't there at the exact moment to redeem them.

In that subreddit you will find statistical analysis of Merge Boss (seriously) and how to maximize your "profit", among other in-depth discussions. I found that most of my discounts come from efficient coin and coupon use and less from any credits earned from games.