The mission Out Of Body Experience includes a short fight with a relatively weak Constructor, which, assuming you are of adequate level for the mission, should be relatively easy to kill before it creates other bots.
This mission can be obtained a short ways last the entrance of the Bloodshot Stronghold, after you have rescued Roland. Just make sure you're adequately levelled, bring a corrosive gun, and aim for the big red eye. This one shouldn't be too hard.
Something to note is that, although the Constructor will have smaller units and the occasional EXP Loader around it, the achievement is only checking for the bots that it actually digistructs, to my knowledge.