Dang, you've made it pretty far with that roster. I'd personally say Herc due to linking stun on his sp2 (have to play a little around the stun command), but more importantly because he has the highest health of your available champions. Just remember that you can't use his immortality. Caiw will also be good with his high block proficiency.
Sunspot and kitty are the only sureshot options I see, maybe starky but I'm skeptical, how did you even manage to get that far with a roster like that anyway?? I'd say just keep going you've already got him to phase 2 and phase 3 isn't really hard honestly since there's no evade charges to worry about or doing the wrong challenge, plus kitty with her playstyle should be a safe bet for the indestructible phase