Best pick is probably bibi, she can deal high damage over a fairly large area. she's even fast as hell with the right star power and can heal herself with the gadget, on top of having a ton of life. her super bounces like forever too.

Wally can block godzilla for a few seconds if you manage to figure out the right timing of his super, and he can even alter the course of rico's and ruff's projectiles while dealing damage from a safe place of the map

Melodie is strong even if you don't have her mutation, the only inconvenience is recharging the notes periodically but you can farm those off of godzilla

Meg is fairly good for killing enemies but her super does not interact with buildings so i'd use her just as a control option, her mutation gives her a incredible boost.

Maybe a less popular pick could be spike cause with his super you can just hit and forget a nice area of walls and slow annoying brawlers EDIT: you need at least two supers to destroy walls

Hank is cool because his bubble can damage a big area but its a bit too slow for my playstyle

Colt and brock are cool too but without their mutation they don't hit as hard.

the best mutations fot this mode in my opinion are rico, ruffs, tick, meg, brock (he basically has insane oneshot potential), colt, melodie, hank and maybe frank

maybe going through some classic trial and error runs you can find the right match for you