During the "For Bella" mission, reach the "Find a way into the Weyland-Yutani Facility to save Bella" objective. Follow the stairs to a door and exit the interior to enter a rocky exterior area with lava flowing down a cliff to your right. Run partway up the path ahead and look to the right across the chasm with a scoped weapon to see a box of donuts. Shoot them and they will disappear. Once done, look to where your teammates are waiting to see a huge donut appear uphill. It will roll down, launch off the cliff, and explode in the lava.
Find the following unique weapons by searching the corresponding locations:
In mission 10, during the objective to destroy the third AA gun, find a container with a rocket launcher inside. Search that container further to find the flamethrower.
In mission 4, while exploring the tunnels with O'Neal you will be stopped by a door that must be sliced open. Leave O'Neal waiting beside the door and follow the path to the left of him. There is a table with the pistol at the base of some steps.
In mission 1, explore the gravity well with O'Neal and reach the top floor, where several aliens will attack. Kill them then search the nearby rooms and catch up with O'Neal. After you are reunited,defend O'Neal from alien attacks while the door is opened. Searcg the side rooms on this floor to find armor and the shotgun.
Hudson's pulse rifle
In mission 5, find the elevator, ride it up to meet with O'Neal, and get your weapons again. On this floor, search the offices to find the pulse rifle.
In mission 9, turn left as you enter the interior of Hadley's Hope. Search the path until finding a room that contains armor and the pistol.
Vasquez's smart gun
In mission 6, you will enter an area full of shipping containers while on your way outside with O'Neal and Bella. Fight through the soldiers then go down the stairs to a garage where aliens and soldiers will enter through the open doors. Clear them out, then search the pallets stacked against the wall to find the smart gun.
On the "The Raven" mission, reach the room containing Boilers. Sneak past them and use the sewage pump switch that will start a distraction and kill the Boilers. Proceed to the generator until reaching a four-way intersection, then go to the right toward the dead end. Get close to the barrels to find a Newt's doll's head. Interact with it to earn the achievement.
During the "One Bullet" mission, continue until reaching the operating rooms that contain Xenomorph specimens in glass containers. Go past the room containing victims with Facehuggers and through the hallway ahead with human soldiers. Locate the door surrounded in green light and search the hallway to find an office with the second audio log for this mission. Do not enter the second room where soldiers will try to activate alarms. Instead, circle around to another hallway leading to the same alarm room. Cruz will wait to rush inside the alarm room. Instead of going with him, leave the operating room through the alternate door and enter the alarm room through the left-most side of that path. You must enter the alarm room using this alternate path in order to see a miniature top hat and cane inside a glass container, a reference to the scene in Spaceballs where Chestburster explodes from a chest and imitates the Michigan J. Frog dance from the Warner Bros. cartoon.
On Mission 6, "For Bella", proceed until dropping down through some debris. Look inside the locker next to the sentry gun to find three spheres that resemble the special stage symbols from Sonic 3 And Knuckles.
On the "Rampart" mission, proceed until encountering the pilot of the alien ship, as featured in the Alien movie. Shoot bullets into its skull until a holographic space battle starts playing over its head.
Get rid of the third AA gun in mission 10 and proceed through the rocky exterior to a destroyed building. You will find ammo to the right of the entrance, and a destroyed floor past that point. Stop prior to exiting this building. You will see a sheer drop past the exit that leads to a big open area. From this position at the exit, look to the far right with a scoped weapon to see aliens spawning above the spotlight. They will then run across the roof before jumping down. Stay inside the building and do not drop down into the open area and those aliens will always respawn. Kill them from the building until you need more ammo, then reload the last checkpoint and repeat the process.