Its either not going to work because of the father's power, work but just make a Hulk and he'll end up making him into a "puny god", or bad guy will win and ice oni will come in and win because protagonist doesn't have access to father's power when mindless. Will then look down on him and probably say he's not good enough to protect the heroine but the heroine will interject stating that he didn't want to hurt everyone, etc etc.
You see the problem with manga like this is the heroine didn't do anything herself.
Just pick a stick and beat the guy he is right there. She is an oni too so classic weak damsel in distress trope didn't apply here.