Walk behind your friends.

Run fast.

Run as though Matoro's avenging spirit is after you and hope the sniper's a lousy shot.

On a serious note, I'd try to observe what pattern he follows for hiding places and tactics, and find the most accurate long-range gun at the party. Some of the bigger rifles have a pretty decent shot so that, if you're able to figure out his fighting routine, you should be able to peg him from a fair distance.

Another thing you could do is have someone who's fast be the distraction so that someone who's good at stealth can sneak in and take him out early on in the game.

There's my two cents worth.