Warhammer 40,000: Space Marine

Did this on Easy difficulty so don't know if enemy variety would be different on harder difficulties. Be forewarned.

This is the end of Chapter 6 where you save the inquisitor and retrieve the "weapon" he mentioned which turns out to be an energy source as well esconced in the reactor.

When you reach the generator room, follow the inquisitor's direction to move to the center of the reactor and fend off the orks until the device is ready for extraction.

The boss ork will initially send in some heavies like shield bearers but after a few waves they will mostly be standard grunts. You can shoot the boss but try as I might, I don't think you can defeat him here (yeah, big surprise).

And as long as you don't activate the release (a panel with a blinking red button and a B prompt if you're close to it), the boss ork will just keep sending more cannon fodder at you. :>

Which leaves an interesting opportunity for some other achievements. I don't mean just the enemy count but also a couple of weapon specific ones like Blast Radius and Chain of Death.

I sucked and ran out of ammo so didn't get them myself but there's a large enough crowd of orks to accomplish this if you have the skill and mind to.

Once you're ready to go, press B at the panel, watch a cutscene, grab the device and you'll have won! Congrats!