walm was gonna hit the healer with the fire and the commander had to use her ice to block it, i guess the reaction from both magics colliding caused it to explode and hit the other two heroes
20%? i don't think that's it, with the way he changes a bit to go away, I guess blocking that heavy dragon slaying flash from ragna left him in tatters. or it could be that like a machine he needs to recharge and was only left with 20% in the tank after storming all those bases and fighting...