Players can only help each other once every 10 seconds.
Normal Strategy
Tanks should be assigned to Juggernauts, as they decrease your armor rating.
Healers should be assigned to Assassins, as they put up a DoT that needs to be healed through.
DPS should be assigned to Marauders, as they don't hit very hard and have large health pools.
Burn your cooldowns early and DPS your little heart away.
When you've killed your assigned target, find who is struggling the most with their assignment.
When helping another player, you can only cast one ability per minute, as you receive a 1-minute second debuff after casting it that does not allow you to assist any longer. Make sure you are using your hardest hitting ability to help beat the enrage timer, or a large heal to a player dropping low.
Hard Mode Strategy
Same strategy as normal.
Nightmare Mode Strategy
Same strategy as normal.
Commando Tips
Burn cooldowns early, but save Tech Override for an instant heal if you need it.
5-second Recap
Tank > Juggernaut
Healer > Assassin
DPS > Marauder
Burn cooldowns early
Use hardest hitting/healing ability when assisting others