You first have to do the Achievement "A Horrible Night To Have a Curse"
After you have done that you will get a charm called Cursed Relic.
The Cursed Relic gives you 1 HP in battle and it randomly assigns you with a weapon every time you press the fire button. But if you hold the fire button down and just keep attacking you will hold that weapon till you release or do something else like dodge
With this equipped you will have to multiple bosses, then the relic turns in to the Divine Relic and you will get the achievement.
It has to be done on regular or expert difficulty
You can also track the progress on the relic icon, after each fight the icons eye should open up more
A Tip to get trough this is spawning Djimmi the Great before going in to a boss by spinning your left stick around, to get 1 extra HP in the boss room. You can also get 1 or 2 extra HP in the fight by doing a parry.