EDIT: It's been over a year now and the game has recieved a lot of updates. I haven't played in a while, so I'm not sure if all locations are still relevant for kills. It has been reported that Vault of the nine works for the beholder. Thanks to
Mince Pie 77 and
The Globalizer for finding and confirming this.
This achievement is unlocked by killing a mimic, a beholder and a gelatinous cube. These monsters are very rare, and you'll only encounter them a few times on your way through the game. Here's how to find them:
Mimic - If you've played Dark Souls you'll know what they look like. They appear as normal chests, but they'll turn hostile as soon as you try to loot it. I encountered several of them on my way to 60, but a guaranteed place to get it is the Clockwork Guild Tomb. As long as you're thorough, you should be able to get this one no problem.
Gelatinous Cube - There's only one of these in the entire game, but thankfully it's unmissable. You'll encounter it during the Clockwork Guild Tomb, and you'll know what it looks like when you see it. He hits hard, so be careful.
Beholder - This is the one many are missing, probably because they just don't know where it is. I discovered that one of the bosses in the skirmish "Aberrant Assault" is a beholder. He's not even the final boss and he's a fairly easy fight with a full group. He's called "Eye Tyrant". The only restriction on this skirmish is that the minimum level to queue up for it is 57, and as far as I know this is the best and easiest place to kill a beholder.
Happy hunting!