Well if you want to be cheap, there's a few infinite loops you can usually get him in. The one I use is just shoot at him from close range like 3 times then DB, and when he's down shoot at him a few times then DB, and keep going. Sometimes you can do this for a while. Be careful when he switches from shooting back to attacking you with his fists though.
Another one you can do is to snatch him then melee a few times, then snatch then melee, then snatch then melee, just don't let him get back on his feet.
If you want to beat him legit, it's kind of hard. (at least for me) You just hafta get used to his attack patterns and dodge/attack at the opportune time.