At the start of the level DEPOSITORY you will come across a big ape looking creature. This is a Librarian. Normally you have to approach these monsters very slowly to avoid massive damage. However for this achievement we just want to kill it.
Your best bet is a machine gun with MGR. If you don't have a machine gun on you you can load up the level POLIS and buy one from the vendor. Now select your MGR and unload on the Librarian. 1/3 clips should easily finish it.