Guild Wars 2

i was with a group and we were attacking a supply camp, there was one npc left – the guild claim person, must have been around 20 of us just hammering at this one guy for ages, because of his buff that made him invulnerable, we could only hit him for a 1 second window in every 2 mins
after a while a huge group of enemies came rushing down the hill and killed us all =.=

You should learn mechanics then. For 5 min after recapture npc will have buffs that boosts its damage and makes him nearly invulnerable to attacks and some conditions. It still possible to kill him with that buff if you stack conditions on him (damage ones have their duration halved) or you can just wait for 5 min till buff wears off.