This is similar to what happened in the late Middle Ages, where plate armor was good enough and widespread enough that the fighting manuals of the day codified the mordstreich, half-swording, and basically judo. With magical protection approximating the same effect, mass combat could devolve into a field of sweaty, mostly naked men grappling with each other, rolling about on the ground. Perhaps as a defense, they would oil up their tanned, muscular bodies prior to the battle.