This one is a No-Brainer really. It's also quite simple. If you've played through the game already in Single player on Normal, it's going to be a bit more of a challenge, but with a second player it should be much much easier.
First of all - Select the Mission "Dawn". This mission is only 5-10 minutes long (depending on deaths) and it is by far the easiest.
If neither of you are very good at the game, I suggest one stays back, while the other runs in and takes out the enemies, and if they die they can respawn on the person staying back. Such a cheap method, but, hey, it works like a charm.
There are mostly Grunts and Jackals here, so any weapon with a range on it or a single shot kill to the face such as DMR, Carbine or Pistols are VERY useful. One of you should also grab a plasma pistol when they're dropped so you can take out the shield of the elites in the later level on the Missile Bay.
The missile bay is probably the hardest part of the level since you can get attacked from all sides if you run into the middle. I suggest running to the left next to the debris as soon as you enter this area to flank the enemies next to the console you're supposed to activate. You can also pick off any grunts or jackals on the bay within your sights.
After you've activated the console, grab the Carbines from the dead sniper jackals if they haven't floated off and keep picking off the Grunts next to the jammed reactor one by one. When there are only a few more left, run back to the right where you entered, and flank the remaining elites.
Once you pop the reactor back into place, the level is over. Easy enough right? YOu've just earned 10 Gamerscore and a bro fist bump from your friend. (Or your second controller, if you have no friends :( )
Please give feedback - I tried to detail it as much as possible, as there may be video guides that explain these better. Though with what I worked with last night I found this the best way.