The video guide below shows how to do this very well. However, there are a couple of things not apparent in the video that would be helpful to know. First, this is a timed event--if you have not killed 8 robots by then you will not unlock the achievement. In that case it is best to simply restart the game right away and do it over, otherwise you may want to wait until the end of the game to try again. Second, your weapon has two firing modes-rapid shot or charged blast (hold down trigger), plus by hitting right bumper, it changes to a melee weapon (not recommended here). Lastly, try to get headshots--you will need 100 of them for another achievement.