I think a damage multiplier (for damage taken or both taken and dealed) by the boss could be a decent and easy enough solution. For every unique player who dealed damage to it, that multiplier can be adjusted. That should prevent both cheating the system by keeping distance on summon and HP jumps when someone new joins the battle. However im not sure what happens if someone joins the battle last second. If there is still a lootbag dropped, a potential fix for that would be only dropping lootbags for players who joined the fight before the boss reached 50?% HP.