Green lasers will allow enemies to be killed with a single shot. 
They can be found at the following locations. 

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. 

Andy's Neighborhood: Find your way to the location with a broken 
car. The green laser should be right in front of you. 

Alleys and Gullies: Turn to the right when you see the first 
airplane box.