I did the same to paladin Leeroy in Tomb of Giants. He took a few hits to stagger, but after that I could stunlock him with claymore 1H-R1 easily.
Something else I discovered is that hitting something while it's doing its death animation still counts as a hit, and has all the same consequences. I already knew the Server's healing still triggered, but it also means large weapons won't whiff, and if attacking the target counts as betrayal, the betrayal will still be triggered. The last one happened to me in the Forest. I was using the ents to get easy kills on the Forest Hunter NPCs, but when 1 of the NPCs died I decided to attack the ent. I indirectly hit the already dead NPC, and suddenly all the NPCs start chasing after me...