When you want to deal much damage to players use a splashpotion of harming.
You use the splashpotion like in vanilla minecraft with a simple "rightclick"
All the better you hit with the potion, as much more damage it will take the player. Armor will influence the damage which accures
You can get the potion at the secretshop for 150$ apiece.
The ID is (373:16396)
You can get damaged from your own potion.