There are plenty helpful threads, sites and documents that rank the characters in the game, tier by tier.
Why not try and collect all these under one topic?
Still trying different opponents, but 4* Widow, Domino, and 3* Iron Fist has been “winfinite” for me so far. I start by adding 2 Purple AP, and after one purple match, Fist can turn the board black, with Domino helping more black tiles fall. Widow’s stun helps with crowd control, and keeps the black below 12 AP to allow Fist to keep turning purple to black. If the matches net you enough yellow, Widow’s yellow can drop AP costs by 2. I get lots of 5+ matches, and to keep the whole opposing team stunned. Experimenting to see if Domino is needed or a better third would make the grind move more quickly. (Note: This is with my two 4* unchampioned [level 210].) Waiting to see how consistent this is before investing the iso to champ two typically lower-rated 4*s.