NPC getting divebombed nonstop everyday by invul zealot-inq and iframe lord bm-assassin is something,
you cant defend against them, they will do damage no matter what or how many cc you do
killing NPC is one of the thing to ensure your team winning the game, but currently it is just too fragile, so my suggestion is to either give NPC high evasion or out of combat hp regen or putting a mechanic where theres a limit how many times you can kill NPC.