Before the battle starts, make sure you equip (and develop) a light machine gun as this will be your main weapon. As your first weapon I would suggest either an assault rifle or a grenade launcher. Decoys are also very helpful, as well as armor (like the Battle Dress).
When the mission begins, you'll find yo yourself immediately in a boss battle, which is actually the whole mission. Your goal is to eliminate 4 Skulls who are wearing armor as well.
The tactic I would suggest (since I beat the level with it) is to, as soon as the mission starts, run towards the near hangar, and climb on top of it. On the right side(seeing it from where you spawn in) is ladder you can use. The Skulls won't come up here, except the melee guy, who delivers one strike and leaves. So this is your cover place.
Also, the puppet soldiers can't grab you here either. Now just spray you lmg into the Skulls.
The Skulls have two types of attacks you'll deal with on the roof:
-They throw a giant rock at you.
-They spawn a giant rock under your feet. These rocks explode after a few seconds, and it'll kill you.
Later on in the game, you'll be able to develope the Brennan LS-46 (pretty sure that that's the name). This sniper rifle is the strongest in the game, and I'd definitely suggest using this one when you come back for mission tasks or S-rank.
That's all you need to know, good luck!