If, like myself, you messed up your new game+ and find yourself having to start a new game from scratch to get this cheevo here is a tip that may help, it certainly makes it a shade easier for me so far on my run.
What to do is, if you have a scythe with Health Steal on it in your original playthrough, gift it to a friend through the serpent tome then have him/her gift it back after you start your apocalypse run (or just get them to gift one of their own weapons), either way you will have a health steal weapon waiting for you.
Granted, you will not be able to use this high level item but you can sacrifice it to the first possessed scythe you collect, as long as gifted weapon triggers the "weapon level up" box, you can then select health steal to go on your freshly picked up scythes, you can continue to do this with every possessed scythe you pick up as you go (this is what I have been doing and it has so far worked every time).
It should be noted that the steal percentage will drop dramatically when this is done and this tactic will not make you unbeatable, certainly not in the beginning, but as you go on the percentage will increase a bit nearly every time you feed a new possessed weapon. What this means is that you will fare much better against standard enemies allowing you to save your health vials for use against the bosses because on an apocalypse run you will likely still need them
Just got this today, predominantly Necro build with Ghouls and all ghoul related enhancements maxed, plus Harvest maxed, only got health steal up to 17% but it was vital on a few occasions, most effective when heavily outnumbered, use after summoning ghouls and enemies are drawn to them then use RB and X to throw your scythe into the crowd, your health will replenish quite rapidly.