KEYES

HALO>SINGLE PLAYER>LEVEL WALKTHROUGH>KEYES

Ideal arsenal: Shotgun and alien rifle.
Best tip: grenades are your best friends in this level.
Hardest part: Escaping after you find the captain.
How to beat it: immediately jump off the captain's platform and run to the left. Open the door in the left corner and the Covenant will stream out and fight the Flood for you. Oherwise, if you decide to fight the Flood and survive (very hard), there will be loads of Covenant waiting to battle you.
Cool thing to try: At the end of the level, you can actually jump off on top of a Covenant drop ship and use it to get you an empty Banshee - skipping an amazing amount of enemies and jumping right to the next level.

WALKTHROUGH

    Let the other guys fight for you whenever possible. It's best to help the Flood than the Covenant. If you have a shotgun, the Flood can be handled.
    You know the routine by now... just keep fighting. When you're told to wait at the bottom of the gravity lift, don't dawdle. Get to the top of the hill as quickly as possible and they'll beam you up.
    When you reach the hangar of the ship, jump downstairs and save yourself some trouble. When you reach the 2nd big room, bypass the 1st door and alongside the fireright. Use lots of grenades and shotguns.
    When it's time to escape, try it yourself a few times. Difficult, eh? Now when you're given the command to escape, jump off to your left (facing the doors), and open the one on your far left. The Covenant and the Flood will fight each other.
    Now, fight your way down to the escape pod bay. Jump onto the alien drop ship (it looks scary, but it can be done). Jump onto the bridge and then jump onto the Banshee. Jump in, and you're set.