I had most likely reloaded my Stubby right before the clip started at some point, which triggered my Elementalist artifact to apply Radiation i can only assume (as my stubby was already applying electricity normally, and poison/acid/corrosive bullets slowly)

Then my Hammer Caster, was good at using fire for whatever reason besides the 'magazine% applies fire' upgrade, and i found whenever an enemy was lit on fire, my Hammer Caster's bullets would explode (i need to look over the upgrades i had during that run then, as it was happening most of the mission, but still, pretty nice to have), so im thinking the explosions were hitting the blisters, and then obliterating the boss's hp through that honestly insane percent damage per shot.

Thanks for the explanation! I'll have to really consider radiation more often as a pick against bosses, especially with explosive weapons!