During the story you can only pick one of the three new contraptions. The other 2 contraptions can you find in the districts (chapters) so that you can get all 3 contraptions after the story. The Constrictor Harness contraption can you find in the Manhattan Nuclear Bunker District near the end of the district.1)
After you pick up the Constrictor Harness in Manhattan Nuclear Bunker there is a good spot to obtain this achievement. For the Übercommander you have to enter a small pipe, left side of this hole is a train waggon, go under there and crawl forward and you will see the legs of the Übercommander and from a soldier, now wait that one of them are close enough to you and then push the melee attack button2)
Another place without playing a whole district is at the beginning of the New Orleans Confinement Wall District. Go left side in first hole after you leaving the hole you see on your left side a train waggon. Shoot in front of this train waggon to get the attention of the enemies here and crawl under the train waggon and wait for some enemies that they come close to you and then hit the melee attack button.