In several levels are red gas canisters with a small pilot light on them. These can be pushed easily by walking into them. Get behind the smart gun and push it as close as possible, then strike it once with a heavy attack (should be a tail strike). You will have a small amount of time to flee before it explodes, disabling the smart gun
In some levels of the Alien and Marine, you can hear the Predator's classic sounds in the background. In some levels as the Alien you can just see the blurry shape of a Predator moving when you enter a new area. There are also Predator Mines laid out in certain areas that can only be reached by Aliens.
To easily gain XP, search for a ranked match in Multiplayer mode. When it says "One player in game, waiting for more players", quit, and go to the main menu. Then, load a Marine mission on the Normal difficulty with a lot of enemies, such as the Praetorian boss in the "Research Lab" mission. Simply kill enemies now to get easy XP. Repeat as desired.
As Alien in level 3, the Queen will tell you to get to the bottom of a shaft with a giant fan at the top. As you get near it, another Xenomorph will be swept away and blown into the fan. If you use your Focus, you can see two vents on the opposite wall. select either of them and you can safely jump to it despite the power of the fan. As you go, you will find more vents to hide in. Use them to make your way down.