Solution taken from Clement_RKG on Steam:
https://steamcommunity.com/sharedfiles/filedetails/?id=32164...There are 2 steps for getting this achievement :
- Buy 'max health' upgrades until you reach n/50hp or higher
- Heal until you get to 50/n hp or higher
Note : You will not unlock the achievement if your current health is under 50 HP, even if your maximum health is higher than 50.