You will need to unlock and equip armor with the Death Barter enchantment. Next, level that enchantment to max level, costs 9 enchantment points. Head into any level on a challenging, but not impossible difficulty for your power level.
You should notice an emerald counter in the top left corner of the screen. This keeps track of the emeralds you have collected and can use toward Death Barter. At max level, using Death barter will cost 50 emeralds. So, collect enough emerald sand then allow the mobs to kill you. On the last hit, instead of dying, Death Barter will automatically use 50 emeralds and replenish your health. Rinse and repeat one more time in the same playthrough to earn your achievement.