Biggun hits minion ranked enemies for 151% their maximum health, so that you can one shot up to 3 with one cast. Minions and Chumps are the two ranks of enemies that will not counter you with combat mechanics, interrupt, blockbreak, etc. Matches are enemies like Punishers and Bowmasters who will not be hit hard by Biggun. Then you have SubBosses and Bosses which also don't get the bonus. It is very situational, and working as intended.
I am quite disappointed. The title of this thread made me feel like I should somehow SHIELD/protect myself from that much damage. I thought it seemed unlikely, but it was worth looking lol