Make the big guns HNMs half instanced like FFXI's void watch
#1 be at the required range from mob when it pops to be able to attack it.
#2 get out of the area and you are kicked.
#3 once mob is engaged new people coming from far won't be able to attack.
#3 loot is giving away in an universal chest.
#4 the more or less people attack it the higher/lower lvl it is
#5 scales up instantly on new contact of people and it's HP bar fills up to avoid last second zerging