The MIMIR Z600 is one of the large mechs you see broken down throughout the game. Eventually you'll run into working models. They appear in multiple areas but I got the achievement in the Unenlightened area on a mech that was by itself. Make sure you have EMP grenades (and the chain lightning ability is good), and weapons with a chance to EMP work great too. It wasn't too hard since it was by itself and as long as you've been upgrading and finding weapons it shouldn't be too difficult to kill as long as you can stunlock it.