Theres nothing inherently wrong with the idea of instant kill/ percentage kills in a multiplayer setting they just need to adhere to certain balance aspects. They already only work on some adds and dungeon bosses, so they wouldnt be too overpowered overall, but their main balance point is they are spammable, give them a cooldown or a rescource cost and theyre suddenly a lot less effective, especially if their success rates scale with enemy health.