As a Marine you’ll have a decent choice of weaponry although ammunition is liable to be an issue. Aim carefully and avoid wasting it on spraying the scenery. You get a handgun as default and can carry either two medium sized guns like the assault rifle and flamethrower or one big smart gun. Always make sure you choose your reload times carefully, take cover and reload between kills because you are extremely vulnerable when you are fumbling to get that new mag in.
Melee Attack – Using the middle mouse button you can pull off a block or shove your enemy back to give you room to fire. Just remember a heavy enemy attack will swipe through a block.
VP78 Pistol – This is your handgun and you’ll never be without it. It is a reliable back up weapon and very handy when you run out of ammo with your main gun. The default fire is a single shot and the secondary fire lets a quick burst of three rounds go. Always aim for the head and the pistol will serve you well.

click to enlarge
ZX-76 Shotgun – Default fire is a single shot, secondary fire gives them both barrels for some serious stopping power. The shotgun is useless at range so save it for close encounters but remember Aliens have acid for blood so if you get too close a shotgun blast is liable to cause some acid splash.
M41A/2 Pulse Rifle – This is the standard issue gun you’ll remember from the films. Primary fire is a burst of rounds, secondary fire pumps a shot from the grenade launcher. Always aim for the head and if you’re going to let a grenade go make sure the target is not too close or you’ll get caught in the explosion.
M260b Flamethrower – Nothing likes fire so the flamethrower can be very handy. Primary fire lets a jet of flame go, secondary fire lets you spray fuel and then ignite to create a fire barrier. This is great for torching eggs and Facehuggers. Xenomorphs don’t like it either but it won’t kill them fast.
M42C Scoped Rifle – The sniper rifle is a welcome addition to any arsenal. The scope allows you to aim and pick off enemies from distance and it is great for spotting hidden foes. Even up close the rifle is handy because headshots are devastating but if you miss the low fire rate and long reload time can leave you fatally exposed.
M59/B Smartgun – This feels like cheating. It may take two weapon slots but the smartgun does all the hard work for you. It automatically targets enemies so you just point in the general direction and fire. The downside is you can’t aim at specific parts of the enemy and your ammo will run out fast.