Always consider your crew's classes when equipping items and weapons. A Flanker and a shotgun work together wonderfully.
Environment
The environment is your friend. Red barrels, flammable oil, and solid walls let you take out entire groups of enemies with explosions or ricocheting shots.
Shots from below
You can suffer the same location-based damage as your enemies, so be wary of shots from below that can cripple a team member for a turn.