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.

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

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