This achievement is for getting your Health level to 99. This should come naturally within your three playthroughs of Normal, Hard, and Very Hard. This one took a bit longer than the Blood level for me. I was at about level 87 when my Blood achievement popped.
You don't really have to farm for this, but if are looking to get extra Life Crystals, try to get a high combo going. A high combo will allow you to choose a method of execution for the enemies. Choose the Life Crystal one, I believe it's Y-button, and you'll get a large number of them.
Also, smash every pot, box, and breakable that you can. Lots of Blood Roses and Life Crystals in them.
Finally, if you have excess money, you can buy extra Life Crystals through the Gift Shop for $1 million a piece (you get 100, but that's a REALLY bad deal, IMO).