Can be unlocked on casual difficulty (adviced to do so).
Since this achievement is quite self-explanatory, I will give you more of an advice than a guide for how to or why this did not unlock when you tried.
Maze might be the best location for this achievement.
Before entering the dungeon make sure none of your cards will heal you, this means check all your equipped cards to have no 'heals on use' or 'chance to drop hearts' bonus effect, if you find any, just switch to another version of the card. Some legendary cards can have other special healing effect (Ambrosia, Cornucopia), make sure to unequip/switch versions.
Just to be sure do not click near the life wells.
If playing on casual or normal do NOT hover the cursor in the space near the healer's. Even if you are in dialogue screens of other characters you still might get healed by his potions on the barrel, if you hover over them.
If you complied with all above and still didn't get the achievement, there might be a bug going on. I played slow and steady several times and was 100% sure I recieved no healing and I still didn't get the achievement, after a few days i tried with 'Leeroy Jenkins' method (divine potion and teleport/charge/leap spell) and got it (May-June 2020)-needs confirmation/might be repared. If you get any information about this, feel free to correct me.