Libido is the 6th layer of the game, and will require a total of 240 eyes to complete (starting from 165). The enemies are not too difficult at this point, but you may need to use gear to increase your hearts just in case. It is highly recommended that if you have any revive tokens, try not to use them this early in the game. They will be far more valuable in further layers.
I also would like to point out that if you own both the Windows 8 and Windows Phone version of this game, there is a current glitch that may cause this (as well as other Layer based achievements) to become unattainable.
-EDIT-
As of October 1, 2014, a patch has been released that has corrected issues with switching to previously completed layers. All glitchy achievements associated with this issue have now been fixed.