The fastest way to acquire this achievement is using a card that increases max HP by any amount.
If you are at full HP and you equip a card that gives +X Max HP, the maximum health pool will increase, but the current HP points will not.
As such, simply equip said card in town, talk to the priest and he shall heal you to full HP, counting progress towards the achievement, then unequip the card to lower the max HP pool. Repeat the process until you get the achievement.