1) If your going after one guy a good dual combo is the plasma gun you can build up and a SMG. Fire the plasma then take em out with the SMG. (this has been mentioned in another thread, but still a good strategy). If there are multiple enemies forget this weapon combo, it takes to long for your plasma to charge up.
2) Pistol and SMG is wicked for head shots and you can normally take out 2 guys with this combo, but 3 guys can be a problem due to the ammo of the pistol and needing to reload.
3) With the shotgun and sword be alot more sneaky, if you see someone coming on your radar hide or begin to move slowly so you can fall off their radar. Once they're close enough then you can attack with these weapons. If your too far away, your screwed.
4) Personally I never use the needler, even two of them take to long to kill one guy (I find anyways).
1) If you see a ghost or banshee etc. firing at your team, don't warn the enemy your coming by firing at him too, sneak up on him (because he is probably not checking his radar) and steal his ride.
2) Sticking together is always a good strategy like a few people have already mentioned.
3) When you have the flag or skull and your being fired @ with no help around, don't just keep running... fight back. (both are very powerful melee weapons).
4) When in games like CTF and Assault, if the original plan goes wrong, regroup and try again. Going in individually (regardless of the time remaining or how good you are) never works.
5) My post is too long sorry ;)