The name of this level is Reign In Hell, it is the same one that you get the Overlord ability and not too long after obtaining it. You'll come across an area where there is a standard guy with a gun across a jump, and then behind him 3 of the exploding spider guys will appear. If you wall run to your left you will see a guy with a shield and a railing you can use behind him. You may have to dash to get to the wall run without the spiders blowing you up, but after getting across just use Overlord on the shield guy and use the railing behind him. I'm unsure whether or not the achievement is cumulative or if it has to be a single enemy that gets 4 kills. After two attempts of using the ability on the shield guy I got the achievement and he should've only been getting 3 kills each time, so it may be cumulative. Either way, this may or may not work 100%, but if you keep using Overlord on him as you go along that path you should get it fairly easily, as I had only tried it twice and got the achievement. And yeah, in the video I mistakenly call the ability Overload, I just now realized it was actually called Overlord..makes more sense I think instead of Overload.