|Checkpoint XP Farming:
During Level 10, when your objective is "Destroy the AA guns (3 of 3)", you'll come across a soldier that's using a flamethrower against an alien and
"Checkpoint Reached" will appear. Down the hill from where you are now you'll find a bunch of enemies. Simply reload the checkpoint as many times as desired and
you'll get up to 6.000 XP an hour and chance to complete many additional challenges for more XP.
Spawn XP Farming:
During Level 10, once you've taken out the third AA gun, you'll reach a destroyed building after you've passed the rocky exterior.
Stop before leaving this building. There's a drop down after the exit door that leads to a big area with unlimited spawning aliens.
Stand in the exit door and look to the far left of the courtyard and you can take out the aliens that appear above the spotlight with any scoped weapon.
Remain there for as long as desired to get and XP and complete additional challenges for more XP. When you run out of ammo, simply reload the previous checkpoint.
Spaceballs Easter Egg:
During Level 7 you'll find yourself in an operating room with Xeno specimens in glass containers. Continue past the test subjects that are attached to facehuggers and you'll end up meeting some soldiers. The soldiers will be attempting to activate alarms in the next room and Cruz will be waiting for you to rush in there. Leave them and try to find the second path into the alarm room through a door with a green light. By taking that path instead you activate an easter egg when you enter the alarm room. Inside one of the glass containers is a top hat and cane, very similar to the ones used by the dancing chestburster that sings "Hello! Ma Baby" near the end of the movie Spaceballs.
Achievement Tips: "Short, Controlled Bursts"
During Level 1 you'll get to a hangar bay after you've fought the first group of aliens. One of your friends will destroy a gunship and then start opening a door, leaving you to defend him. Two AI allies are standing behind a barricade and to get the achievement none of your enemies can cross over that barricade. Standing infront of the barricade will make the aliens go after you instead of jumping over the barricade.
Achievement Tips: "Easter Egg"
During Level 2 you'll reach a dark area with facehuggers right after you've opened the door with the loader. Kill them and then look for a blue glowing switch. Press it and a door will open in the previous room and inside are four painted alien eggs.
Achievement Tips: "Fire Drill"
During Level 3 a timer will start at 2 minutes and 30 seconds the moment you enter the hangar at the end of the level, To get the achievement you have to press three switches and then enter the dropship before the time is up. Playing on Recruit and using a shotgun helps a lot. Back-up your savegame if you think you might miss the timer, otherwise you'll have to replay the entire level.
Achievement Tips: "Heavy Lifting"
Start Level 5 on Soldier difficulty or higher. The goal for this achievement is to kill the boss at the end using the powerloader in under 1 minute and 10 seconds. Keep forcing him into a corner and mash the LT button take him out with the quick punches. Should you miss the timer: pause the game and reload as soon as you jump out of the powerloader or back-up your savegame beforehand, otherwise you'll have to replay the entire level.
Achievement Tips: "Mostly Come At Night..."
During Level 5 you'll meet the sound-sensitive aliens when you're looking for a way out of the sewers. Flip the switch that powers up the generator and they will die. Not far from that area there are three barrels near a yellow light. On top of the barrels sits Newt's doll.
Legendary weapon locations:
In these six locations you'll find the hidden weapons that belonged to characters from James Cameron's Aliens.
The legendary weapons cannot be upgraded, which makes some of them a little worse than fully upgraded standard weapons.
During Level 1, when you're exploring the gravity well, you will reach the top floor where several aliens will attack. Don't meet up with O'Neal just yet, explore the side rooms on this floor first to find some armor and the legendary, which has a faster rate-of-fire and improved firepower.
During Level 4, when you're in the tunnels with O'Neal, you will come across a door that O'Neal stands next to until you cut it open. Ignore him and take the path to the left of him and on the table at the base of the steps you'll find the legendary, which has improved firepower.
Hudson's Pulse Rifle
During Level 5, after you've gone in the elevator, you'll be going up to meet O'Neal and get your weapons. After he opens the door for you, go straight ahead and check out the offices to find the legendary on the floor, which has improved firepower, greater accuracy and doubled clip-size.
During Level 6, after retrieving the enemy personell at the end of the level, you'll encounter some soldiers in an outdoor area. Once you reach the next building, go down the stairs and open the door. Look in the left corner of the room to find the legendary, which has improved firepower.
During Level 9, just as you get into Hadley's Hope, turn left, and explore the path to find a room with some armor and the legendary, which has a faster rate-of-fire and improved firepower.
During Level 10, when you're trying to destroy the third AA gun, you'll find a container with an RPG inside. In the same container you'll find the legendary, which has improved firepower.