Online play can be seriously difficult for those skipping the single player mode and going straight to multiplayer on Xbox Live. Experienced players seem to take a lot of hits while they shoot minimal bullets to kill you. Try the following strategies.

Get the Kevlar Vest for 100 points for extra protection.
Get five helmets for 20 points each, protect your head.
Get the M16 with grenade launcher or the submachine gun (RP-46) for maximum damage with minimal shots.

Do not stand still; move around and shoot to avoid direct hits. Also, aim for the head.

If you are low on health and body protection, detach from engaging and get your armor and health back up quickly.