Just got this so I thought I'd give my two cents. First off this is easier than the other ones and faster too. Make sure you have that famine skull equipped!
Here's a quick run through of the way to succeed in all the levels.
1st mission - You start with active camo use it! You can run through all the way to the end without killing anyone until it says to kill them off. Here when it says that, they walk in from the top left and you can spawn kill nearly all of them with plasma grenades, you should have no issues here.
2nd mission - EDIT from Shelby:There is a ghost at the bottom of the Mission 2 map that makes the mission a lot easier! (If you do do this, be extremely careful of the top right side as there is regular plasma cannon fire that could spell gameover not in his comment)
Defending the engineer can mostly be done using melees. At the bottom left of the map is the regen field power up. For the half of the timer use sprint and melee everything, then switch to the regen field and melee some more. Save your focus rifle for the heavy enemies at the end, particularly the brute chieftain. Bonus, you can heal the engineer with the regen field.
3rd mission - The trick here is to use your camo to take out the jackals in the bottom left, and then hug the right side of the map near where you started and hijack the wraith. From then all you have to do is look after your wraith and rake in the kills. Enemies spawn from the ship in front of you, up north and bottom left. You will probably have to get out and kill the camo elite after everything else is dead. It will seem like there's a continuous stream of suicide grunts but there isn't. With the wraith you don't have to worry about ammo.
I actually got gold on this with minimal effort.
4th mission - Here you want to sprint up the left side and get the camo by the ghost. Then you want to jack a wraith using the camo, you want to make sure you save that ghost as you will probably end up losing your wraith and its a free ghost for when your wraith is nearly dead. Speed through killing enemy wraiths until you can go no further in a vehicle, from here, you'll want to play dodge the fuel rod shots and take out the lieutenants one at a time (the other 4 are easy to take out with the wraith). The last lieutenants take the bait and go down pretty easy, job done.
5th mission -
EDIT: Shelby went with a sniper rifle for this mission, if you've got xp to spare and struggle, give that a shot!
unlike 2, 3 and 4 you don't have a vehicle to abuse infinite ammo you do however have a box with an assault rifle, make sure to pick up the assault rifle when you're running low or even at the start, and you'll find if you hug the box it will keep filling up with ammo. You may need to pick up and drop the shotgun in front to get it out the way. 1st part, kill the initial wave and then spawn kill with grenades the elites and jackals coming through the north door with grenades. Once you've done this twice its a boss fight! - 1st form you want to dodge his plasma shots, you'll want to make liberal use of sprint. Just whittle him down until he runs back top, he'll split into four, try shooting all of them until you hit the real one to make the next bit easier. He'll then be an invisible sword elite, the trick here is to pepper him with shots while he's close but only enough to take one hit and then sprint until your shield is back. At this point jackals are spawning in so I'd take them out if they become bothersome. 3rd form is much the same but he has a hard light shield, if you rush him he acts like 2nd form and actually goes down quickly, be wary of numerous jackals and 4 laser turrets. Once he's down be sure to press X to pick up the pistol on his body and enjoy your completed challenge :)
Please give feedback if you think I can improve and vote :)