It seems that the game only drops health potions when your inventory is not full of them.
So a tip: If you tend to not use health potions much and often have 20 potions at a time, your pack is full, so go into your Inventory to the Consumables tab and destroy 5 of them (highlight the item and press
). Then they'll drop when you kill groups of enemies, especially the uniques and bosses, and you can repeat the process. They also tend to drop from chests, even small ones, so be sure to open any you see.
Just to be clear: Only Health
Potions count for the achievement. Antidote Potions, the red bottle with wings, and Polarized Potions, the red bottle shaped like a lightning bolt, do not count
A good farming process can be had with a high-level character going back into the early stages with lower level monsters. Edgewood Bluff, Fields of Unrest, and Firebelly Village, for examples, are all areas that drop Health Potions and have weak enemies. You won't get any XP, but you'll still get progress toward picking up Health Potions, Gold and killing a million enemies.