I love the idea of mobarena, but the fact it has classes.. is no good! I want my players to either band together or take each other out in the arena VS Mobs.. Flip of a switch/button push and the mobs get to dropping! Waves would be nice, 1 zombie first wave, 2,3,4,5,6,7,8,9 and so on.. once the arena has 0 people in it due to death[or the switch was flipped again] the waves stop.
Easy fix. You still use MobArena but you make one class and it starts with nothing. As for friendly-fire, I only know of making that work with factions because of warzone claiming. Although, if you're currently using 1.2.5-R1.3, it won't work with latest build of mobarena.