Don't get cornered behind the bunker. LOL. Try to anticipate when it's going to happen before it happens and find the most secure location you can before it does.
If you're the last man standing and surrounded all you can really do is keep the paint flying as best you can. Lots of snap shooting....come out high, come out low always change the spot where you come out because you know a guy will have you tee'd up. Don't look in one direction too long because as you shoot at one guy the other one is moving up to get you. Use head fakes to get them to shoot at one spot and you come out in another quickly after the fake.
Here is one trick I like to do a lot and it works well. Let's say you have a guy coming down your right, and one coming down your left. You pick one guy, the right hand guy for the sake of this explanation, and snap out on him. Battle until you have him taking cover, then lay a quick stream of paint on him , step out that side a bit(as you continue to shoot) and wrap around quickly to shoot at the guy on the left. Sometimes you can catch that left hand guy moving while you shoot and he won't expect you to shoot at him. Shoot a few and then quickly change back to the guy on the right and shoot at him to keep him pinned. Basically, you're doing what's called "Living behind your gun". That means, using your firepower to control your opponents and make them do what you want. This is how you open up opportunities. Now obviously if you're new, gun fighting can take a while to learn. You have to get comfy with your timing, and knowing what you can and cant' get away with.
You can use any variation of tricks like this to get out of bad situations. For example if you wrap around to shoot the left guy...and he also takes cover, that gives you a second to quickly move(while both guys are down) and go quiet. They'll lose track of you and you lie in wait to take them out. Or.....even with good snap shooting skills(which take practice) you can often take one or two guys out just on gun skills alone.
Another trick I like to use when I'm pinned is to yell at an imaginary team mate to get his "gun working quick and take out that guy right in front of him". Mind games work great in paintball and I've actually won a tournament 3 on 1 using this trick. They were so convinced a guy was left they held up a few seconds which let me take them on one at a time.
In the grand scheme of things, if you're out numbered and pinned down in one spot, you're probably not going to win, however that's never a reason not to try. If you give up you always lose. If you put up a fight, you'll be pleasantly surprised once in a while and walk away with the V! Not only will it feel awesome, but the next time they have you trapped they won't be so sure of themselves.