I'm sure you're going to be running something more than once. Simply try to remember how much attacks you can on a boss before you end up pushing phases, then use the buff on the last of those turns the next time you fight it again. Like say you can do 4 turns of regular attacks on the stunned target before pushing it out of stun, just use the stun damage up skill during the 4th turn or 3rd turn to be safe. You get your time to recover and still deal damage.