In Halo 3 Multiplayer it is the craft of aim that wins all battles, that and familiarization with the levels and its weapons/perks. My suggestion is find a control set-up that works for you (i.e. Bumper Jumper, Default, ect.). I jump constantly so I use bumper jumpers and play using most of my fingers. Jumping constantly throws off your opponents aim, especially up close. Melee is also key, so as you jump and shoot your opponent if you have the opportunity to melee without putting yourself in a emminent danger, take it.

Also weapon strategy can give you the upper hand. For instance, a frag grenade bounce and a battle rifle/pistol shot to the head is instant death even with shields. Another instant kill without using the rocket, sword, hammer, assassination, or sniper rifle headshot; would be what online players consider the "noob combo" as made famous by Halo 2. A Plasma pistol duel wielded with an SMG or pistol. Charge your plamsa pistol and release the charge on your opponent, taking down their shield completely, then finish them off with any combination you see fit.

It takes a bit of strategy and personal preference of weapons to become "beastly" at this game, but even the worst players get better with practice. Good luck to you.