It's all about witts n skillz
I'm not the greatest 1 on 1 player in the world but here's my 2 cents:
1. If it settles down, it is a battle of witts or skills. If you know one is better for you than the other, turn the game the way it suits you:
Skills - Get dug in and snap-shoot. Don't let up on him and force him to play on your terms
Witts - Confuse him in as many ways as possible:
* Shout "S*** I'm out of paint" "Where's he gone" "This might hurt a bit" or anything to put him off and make him think things that are untrue.
* Do unexpected things: I was playing in a 1 on 1 with a team mate, and he was behind a low stand-up and he'd come out (my) left a couple of times, so I tried to put a couple of balls right at the correct time. As I am doign this he comes over the top and nearly had me bang to rights.
* If you have plenty of paintballs then consider throwing a handful at/over his baracade, this may confuse the hell out of him at the right moment.
When he is confused, move, get an angle on him, then light him up.
Keep your head, he is likely to be doing all of these things and more to you.
Richard