An example from my own playthrough:
I buy every defensive upgrade possible for the main character and have her at maximum HP. The enemy summons two more enemies, one of which flame bursts her, then the main enemy chain lightnings on her, and another attack before I can make any action kills her. Lacking any form of revival at this point in the game it is simply over with 3 against that fight.
You can debuff the main enemy, but considering there's no way to do the same to the summons it seems to be simply the kind of game that will just swing to kill you very frequently with nothing you could have done differently.