It varies from game to game, but for the most part I just pop my head out unload a shit load of ammo in whoever's face I see first, hide again, reload, and repeat. Sometimes I get bored and just run around gunning down everything with a pulse. Fire is always a good choice too.
My best friend is hilarious. He cannot shoot for shit, like seriously he can't get a head shot on a still target 5 feet from his face, but if you give him any explosives or fire he will turn into a fucking monster. If you see him with a rocket launcher or explosive, you'd better run because he will fuck you up. I think that probably stems from his lack of regard for his own life though, because many times he has backed me into a corner and dropped a grenade at both our feet and just stood there blocking my escape.