During the Marine campaign when you have to escort Tequila to the security door, she may not unlock the door. If this happens, it indicates that not all Xenomorphs have been killed. Walk around and look for them hiding on the walls, or a facehugger crawling on the floor.
You will get Alien Vision once the Elite Mask is obtained. It allows you to see the Xenomorphs. The mask is near the end of the campaign, inside the pyramid on a large pedestal.
You will eventually obtain the Combi-Stick in Campaign mode. The Combi-Stick is a ranged weapon that retrieves itself after a short time. It is slow moving compared to other projectiles, but deals massive damage. If your target is not killed by it, it will get pinned to the wall for you to finish it off.
While playing as Predator, in the level after getting the Combistick you must enter a building called "Security POD 1. You are told to cut the power, and the objective marker is on a recharge station. Simply punch the small power box above the recharge station.
Shortly after the start of the game, you will receive a mission update marked "Enter The Compound". Set the dead Predator to self-destruct and kill some Xenomorphs, move close to the doors. Look up and find a jump point on top of the wall above the steel doors. Jump up there to enter the compound.
When escaping from the collapsing refinery, one of the objectives is to "Smash the pipes to put out the flames". Drop to the ground and look into one of the tunnels that are on fire. On the ceiling is a pipe you can hit with a Smart Disc. You only need them to hit it once, then retreat to the large circular light area and hold off the aliens until you get a message to get out of there. All but two aliens will retreat, then you can go through the passage with the flames.
Reach the indicated Rank to unlock each Skin.