The flash mob event at the Skywatch loot cave is intense. It spawns once in a while and can wreck you. Basically Hive and Fallen come in many waves. It can be a handful for 3-4 players from what I remember. That was long ago, so maybe it's easy now (much higher level now). Edit: the Vex/Fallen flash mob on Venus by the Fallen crashed ship. That mob spawn goes off. I got wrecked there once. Four Minotaurs and three Captains were all over me...along with 30 other enemies.