Your job is to engage raiders who attempt to get out of combat to
regenerate others.
You should have high hp, a regen buddy,
and know when raiders are getting out of combat (rescue messages
are a big hint, as is looking around the room).
You should also be among the first to attack when a rush of raiders
comes into the room. You see the guard message (insert guard message here).
You should immediately put your backstabbing skills to use and backstab that guy. Hopefully, that will take out
a good portion of his hp. At this point, you will join the random melee, aiming, rescuing, and such.