All 4 of these enemies are conveniently available in Act IV Chapter 1. I recommend playing on Beginner difficulty if you just want to get this out of the way.
EDIT: Video courtesy of Ham Man 89
In the following order:
1) You will encounter the Warden
in the room where you have to deploy the beacon to kill the Swarmak. Be sure to leave some Swarm alive around here so the Warden has something to kill. You'll have to shoot of his helmet before you can ask Jack to hijack him.
2) The Stump
is in an open area you have to clear of enemies before making your way to Paduk. Just hijack immediately when you see it, and there should be enough enemies around for it to kill.
3) The Pouncer
is pretty hard to miss, you'll find it right in the area where Paduk is. Just look over the edge where it jumps down and hijack it. These things are pretty aggressive at finding a target and will usually kill an enemy in one hit.
4) The Elite Hunter
are those annoying things that cloak and fire Torque Bows. Once you get to Paduk, there'll be a brief pause and you will defend the area from these guys. They might take a little while to get a kill since they have to charge up their weapon while flinching from getting shot at, but it should happen eventually. If it doesn't, just reload the checkpoint so you don't have to wait for the ability to recharge.
There is achievement tracking for these, and it should go up by 25% for each. There was an issue with tracking due to server issues, but I booted up the game today and it popped on the main menu.