You'll need a lot more then that Gunner talent (20% reload speed). Rangers get Swift Aim Modal (20% recovery speed bonus, 50% reload speed bonus). Chanters get a phrase that gives another (20% recovery speed bonus, 20% reload speed bonus). It'll also help if you keep them unarmored, though that's just with the recovery speed.
My party has 3 rangers and 1 chanter singing that song, they're pretty good at mowing things down 1 by 1. Casters often go down in 1 volley. They're even better when I can get their pets attacking their target.
In order to deal with large group I really have to use chokepoints with my tanks or keep everyone but tanks back in order to get the mobs to concentrate on the tanks.
Biggest problem I've had so far were groups with Ogre Druids, those guys do painful AoE that goes through armor and take forever to die.