I remember looking into Bad-el as a CB champ but I struggled to figure out how to gear him, he seems like he can heal through damage output on his A1 which would make all-out damage output his goal, or simply through his continuous heals which would mean a more traditional hp/def slant.
My main concern is always watching the 10 debuff limit.
Bad-el in place of banshee would make sense how you described it, 20% more damage when the target is under poison is just 5% shy of a weaken. And the number of debuffs placed would roughly even out, there. But Banshee is prob my top damage dealer at this time.
Could he also make sense swapped in to Fatman's place? Fatty's hp burn is redundant because brogni's is placed consistently. His poisons will be less crucial once i bring toragi in. So he ends up being a dec def guy, really, and an occasional ally attack damage bonus.
If I brought in bad-el in place of fatman, it would keep the poison numbers constant, keep banshee's 25% bonus damage, AND add bad-el's 20% bonus damage. Would that make sense? The only thing I would be lose is the dec def.