Best solo loadout for citadelle boss??

So for some context I’m a very experienced zombies player and can do the actual quest quick and easy. I struggle with the boss fight though on this map. I tried the terminus Strat and went in with Packed GS45s but felt like I was doing nothing. I tried an ASG build seemed to do better but still next to nothing even after his armor broke off. Am I missing something or is this guy really just that darn tanky?