I’d recommend putting together a team with the heroes you are focusing on leveling. I find it helps me organize what I am doing very well. I use team 2 for that, as it shows up pretty highly in the order. I find not having to think about “which one am I working on” while on the phone helps. You could also do this for further teams with second priority heroes,etc, but I use a spreadsheet for that.
I’d focus on the following in each color:
Yellow: Finish off Gretel, then get Rana to 2.60 and park her there. Then work on Wu. He helps massively with titans. Then Guardian Jackal, who helps massively with purple titans.
Green: Muggy #1. I’d be willing to feed one of the duplicate Muggy cards to him, but not more. You may want a lot of him, for sheer amusement value. Also, he’ll be easy to finish off. Then re-evaluate, I like Kashhrek & Little John, but you may have other options by then.
Red: Finish up Guardian Falcon to 3.60 (or 4.70 if you can) then it depends. Other than a 2nd Boldtusk, I could see doing any of the reds at the end next. I’d lean Kellie or Namahage, but it’s up to your play style.
Blue: If you can get Kiril to 4.70, great. If not, get him to 3.60. Then Boril to 3.60 and max out a Gato in some order. Again, you can feed a couple Gato’s to the first one as needed.
Purple: Sabina to 3.60, or 4.70 if you can. Then, Seshat to 3.70, since she’s already ascended. You can get Seshat to 3.70 before taking Sabina to 4.70 if you prefer, and if you draw Rigard, he fits in as the #3 option.