It helps if you use the normal gun switch setting. 
Highlight the shotgun and push a to scroll forward 
through the weapons. When you get to the tek bow stop 
and cycle backwards through the weapons using B untill 
you get to the Tek Bow.

Now pull out your Tek Bow and shoot an enemy as 
fast as you can. The enemy will fly up as though 
you have shot it with a scorpion launcher. When it 
falls down it will be dead! This trick also works 
on bosses and in multiplayer.