This achievement isn't difficult at all. There are 2 different ways you can go about doing this.
1. If you're playing with other people you can all shoot the Constructor at the same time and it should die relatively quickly. (This is how I got it, playing with 2 friends and having the Constructor be 2-3 levels above us. We didn't have to much of a problem with it besides maybe having other enemies shooting us at the same time.)
2. You can easily do this by yourself if you have a higher level character. Once you've leveled a lot, load up your character on
Normal Mode and you'll be able to kill one within seconds of it appearing.
Constructor Locations:In
Thousand Cuts, a Constructor will appear at the bottom of the Hyperion base, while a Badass Constructor will appear near the top.
In
Arid Nexus - Badlands , a Badass Constructor will spawn at the entrance of Hyperion Info Stockade, while two Constructors will spawn at the top once mission Data Mining is ready to turn in.
In
Hero's Pass, a Constructor will spawn near the middle of the path, while a drop barge will drop a Badass Constructor near the end, in front of the entrance of Vault of the Warrior.
In
Bloodshot Stronghold (side mission - Out of Body Experience), You have to put an AI into different robot body types. Constructor will be the first one. (Thanks to
LOUIS 1886 1982 for the information.)
For those of you wondering what a Constructor looks like.
http://vignette2.wikia.nocookie.net/borderlands/images/5/53/...