Failing to break the circles will cause the boss to retaliate with an AOE attack, dealing heavy damage.
Breaking the circles with knock the boss down (the first time) and cause a basic interruption every subsequent time the circles appear.
The only unit that can cleanse right now is Cocoa, but honestly it’s better to use good healers over her for this fight since her kit is a lot more niche even as we take the poison mechanic into consideration.
The boss does not have a ton of basic attacks that you have to worry about, breaking the respawnable poison sacks to avoid some mechanics and avoiding the stun prevents a lot of the issues of this fight.
Ultra itself has a lot of HP, but a lot of it is easily dealt by the fact that body parts deal damage to it when broken, this fight can easily be done below power without a healer as long as you are capable of dealing with the circles and attacks that can be covered.