Nope, blowing off parts like the wheel and Fallen Walker's legs will always do a % of the boss's health. It just happened to do 32k because Nightfall has higher health than say, the level 22 version--thus the % damage you deal by blowing off a part will automatically be higher. Arc has nothing to do with that spike in damage, what Arc IS good for, is blowing off parts faster from base damage increase. Just explaining the nuance of the mechanics, obviously bottom line is, while Arc Burn is active, use Arc.