Waterfall Jackal

On Heroic, if you've done your hero stuff with the hilltop survivors and then you head for the rockslide area, you may find a lone Jackal standing around between the waterfall and the tunnel entrance to the rockslide area (sometimes you get an Elite instead). You can easily creep up behind and have some fun.

For starters you can watch his movements as he jerks his head around in bird-like fashion. When you've had enough of that, try suddenly moving around in front of him, causing a highly amusing delayed reaction of surprise. Or, take a headshot from behind to see the death animation up close. Superb stuff. I'm sure you can think of a few other evil things to do as well. A couple of plasma grenades to the feet should see him soar up high. Join the birds, beak-face!

Conveniently, there's usually a checkpoint as you approach the Jackal, so you can keep reverting to repeat the fun.

As for other difficulty levels, on Legendary you don't seem to get an enemy here, whilst on Normal or Easy you instead get a group of three enemies formed from Jackals and Elites (never more than one Elite, I think).