Hyper Light Drifter

I got this in the West zone. In the first area with the large groups of trees, and a skeleton you need to pass under to progress to the next area. In the NE section of this screen, there is a large group of Dirks (see the Dummy achievement for what these look like), and they can easily be grouped up by walking around in a circle and letting them follow you. If you kill a few, an even larger number of them will spawn.

There is also a great area in the underground sections of the south where a large number of the small robot spiders will spawn. They look like this:

Literally about two dozen of these will spawn at once and swarm you. They only have one health bar, so they are even easier to kill with the bomb than the Dirks.