Been doing some testing in lvl 80 area with lvl 13 ranger, it appears that you only have to hit them once, while someone else in your party does enough damage for you to get credit. I’m dealing 0 points of damage myself (1 with crits) with glancing and immune popping up on monsters in this area, and i’m getting loot and exp as long as someone else in my party does enough damage for a tag. Haven’t tested if the amount of damage they need to do for a tag change depending on how many party members you have