HALO CE & HALO 3 on Legendary vs. HALO 2 on Legendary

You say you're not fast enough... get faster. It's truly (I believe) the only feasible way to beat the level, and indeed, the game, on Legendary. If I can do it, so can you. Try popping out from your cover--preferably a crate or something like that--aiming at a certain Elite and firing a charged bolt from the pistol, going back into cover and switching to the BR, coming out the other side of the cover or jumping over the top, and getting a quick headshot on the one you hit with the plasma pistol.

Also, kill all Grunts before starting on elites, because Grunts can do just as much damage as elites and sometimes more (if they charge up their plasma pistols.)