Without giving away to much, you dropped it there near the end of the game.
If your stuck on the spaceship, try finding a Needle Rifle or DMR and a Plasma Pistol. The overcharged plasma pistol shots should take out the Elite's shields, and then finish them with the Needle Rifle (or anything really). If yoiu still haven't beaten the part with low-gravity....DONT JUMP!!!! While your in the air your an easy target. Make your enemies come to you, and pick them off as they approach.
If your all the way to the cockpit, where there are Elites with swords and Concussion Rifles, try to assassinate a few of the Elites before the rest realize your there. This is another situation where you should rely heavily on the Plasma Pistol. Shoot the Elites with the best weapons first, and look out for invisible guys. After you kill one or two and get hurt, run back to the door you came through to recover...then repeat.