Remember, this unlocks when you complete Legendary. It's probably best to skip this difficulty entirely. Complete the game once on medium by yourself, so you know where all the encounters are, what types of enemies are next, and how to use the new weapons and vehicles. Then a grab a buddy or three and beat it on legendary.
In the Halo 3 guides, Hexxas said: "The Needler explosion is your friend when fighting tough enemies, especially brutes. If you stick 7 needles into an enemy, they will explode. It's an easy way to conserve ammunition. Keep in mind that the elite brutes that use grav hammers are immune to needles, as well as plasma grenades.
Overall, fight defensively. Use cover whenever you can, and don't be afraid to break off and retreat when things get heavy."