Green lasers allow enemies to be killed with a single shot. They are found at the following locations:
Level 1 (Andy's House): When you go to the garage, find a way to get up to the van. When you get to the top, you will see a green laser.
Level 2 (Andy's Neighborhood): In the yard with the torn up junk car, the green laser is beside the laundry post.
Level 3 (Bombs Away): Not available.
Level 4 (Construction Yard): In the open chest, near the yellow generator trailer.
Level 5 (Allies and Gullies): Between the first two trash cans to the right of the level entrance point.
Level 6 (Slime time): In the middle of the alleyway. To obtain this laser the Boss must transcend back into his trash can four times for it to appear.
Level 7 (Al's Toy Barn): On the farthest wall form the entrance point, slightly to the left in front of the "Monster Hockey" box.
Level 8 (Al's Spaceland): Directly in front of the first aisle, which is slightly to the left of the level entrance point.
Level 9 (Toy Barn Encounter): Not available.
Level 10 (Elevator Hop): Directly in front of the entrance point is a vent. Using the grapple hook power-up that you received from Mr. Potato Head. Slowly find the way to the top avoiding the various hazards. The laser upgrade is about halfway up the ascent to the top.
Level 11 (Al's Penthouse): This laser upgrade can be found in the lobby area, which is almost, if not directly, across from the entrance of the level. The laser can be found on the highest shelf, farthest from the stairs.
Level 12 (Emperor Zurg): Not available.
Level 13 (Airport Infiltration): The green laser can be found in the room with all of the airplane displays and the teeter-totter, directly above the puddle of oil.
Level 14 (Tarmac Trouble): The last laser upgrade in the game can be found between terminal #8 and terminal #1, close to the green slime.