This bosses are suposed to be so hard then players need to work together to kill them. It make no sence that only summoner and the player who get last hit get a kill count because it almost alwasy will be the whale that get the kill count. Whale will normaly get last hit and most dammage so free players will have 0% bonus EP.
Free players will be best of by killing normal bosses and just ignore lightning bosses imo.