Here is a way to get several achievements.
You can get:
Flash! King of the Impossible - Lock on laser
Short Circuit - EMP emitter
Trick Shot - rocket launcher
Two Birds with One Stone - LFE gun
Robots Tend to Blow Up - Hand grenade
To do this go to act 1-6. You will run into an enemy called the Jelly fish. Shoot it dead with normal gun, it will spawn into several little bots. Kill them with the weapon you are trying to get the achievement. If you do not have the weapon, check the numerouse weapon boxes that are in the tunnel. Its random and if you do not get the one you want, just reload check point and try again unti you get it. The EMP emitter only stuns them, you will still have to shoot them dead after stunning them to get the achievement. Easy.. :)