Each "chapter" has 3 different stops, each that go 12 star levels. If, say, you was looking to obtain Polly cards, in her chapter (1) you can beat the 12th star of each stop, obtaining her card. After that, you will need to progress to chapter 5 to again beat the star levels and obtain the prizes.
We all know the difficulty higher up becomes really over powered, so its less likely your gonna be "evolving" a character card past the first evolution. If you have purchased lots of packs and maxed out your cards, you may have a chance. But like i said before, the AI now has maxed out legendary cards. If your not getting your first card fused in your second round, you probably wont be winning.
In my opinion, the only lava "hero" card you should use is Lisa. The others dont have great support abilities, and even if Polly was maxed out, she is weak compared to a level 6 blue epic card. My opinion
Also, i checked and the gem gacha is now 50! IDK if its based on time of day, which day, or if they are still testing prices, but this is great news if you got any gems (if your saving up for that)