This mission is awfully frightening at first glance. However once you start taking down skulls and your weapon grades get higher / more effiecent it is very simple. For this one just take a machine gun In my opinion. I used a nice LPG-61. Key points is to know that penetration is for armor. These are armored skulls. Next would be damage so the machine gun offers a nice balance of both. You should own them with this tactic. Make sure they don't kill code talker and extract at least one of them.
100% Eliminate the skulls.
100% Extract Codetalker.
100% Do NOT get grabbed by a zombie soldier.
100% Extract a Skull.