The Gun Pointed at the Head of the Universe
This starts with you immediately having to shoot the floating guys. Run around the ring until you get to one of the shield-things on the side wall, and then duck behind it to shoot at the floaters with the shotgun. It should take 3 hits each and you should be protected between shots.
When they're dead, Cortana wants to destroy Halo. She thinks finding the Pillar of Autumn and causing its fusion reactor to go critical will do it. She thinks about that. In the meantime, open the blast door. The Covanant are there. Shoot through them, and then get to door 2. Nothing behind it. Door 3 opens to the outside, snowy world.
Cortana pipes up that there are 3 generators nearby and if you destroy them, it might distract the locals. She marks #1 for you. Go up the ramp to the top to grab the sniper rifle and stuff. Take out most of the nearby group from the second floor, then head down the ramps to finish the rest off.
There's health and rocket launchers at the last level - blow up the enemies and then make a run for the banshee. Fly it up to the marker. There are only 3 guards there, but be careful not to fall off!
There aren't any guards inside. Just walk in, she says your shield will blow it up, so walk into the light. It'll set off the first one. Run out so you avoid the floating yellow guys. Back into the banshee.
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