Likely a dumb question regarding Magister Sang during the Armoury quest

[Mograx]

Yes I get that he can't be saved. What I simply cannot find anywhere on the internet is how to kill him. I'd rather not pass on free exp since he can't be saved anyway. I've told him on all 4 characters via dialogue that I will kill him, but then the convo just ends. I try attacking him and his armor regenerates before I can do any actual damage.
What am I missing here

[Chaoslink]

You stopped attacking.