Iron Crypticle

As noted in the roadmap, hearts don't disappear, and also as noted, playing on Easy means more hearts will spawn. I would say I was easily getting two heart drops per room. Considering you need to go through ten rooms per floor and four floors, the only thing stopping you from getting this achievement is actually taking damage, since you can't pick up a heart if you have full heart. In my experience though, this achievement will come naturally during one of your early playthroughs where you're still getting used to enemies and taking damage more often. I unlocked this achievement on Floor 2 of one of my first runs.