The Binding of Isaac: Repentance

You need to make 3 deals with the devil in one run. A deal with the devil is made when you trade hearts for an item in a devil room.

I would recommend playing a character with, at minimum, 3 health. Maggy has 4 to trade away but i hate playing her.

The simplest way to get this done is going to be by playing Greed mode. In Greed mode, there is a button in the middle of the room. That starts waves of enemies, you will need to fight through each wave until the button turns off and doors open. Then, you can click the button again to start the bosses, where you will fight 2 (as waves again)

Go up into the shop and see if you can use any of your money to buy health upgrades. Do so if you can.

Then, the button will pop up one final time. If you step on it, you will have to fight one more boss. Killing it makes a devil room appear guaranteed.

Not all devil rooms have deals, sometimes there will just be like red chests or something. These are unhelpful for this. Sometimes, you can also get Angel rooms, you might as well restart as soon as you see one.

But basically, take as many deals as possible. If you can grab two on the first floor without dying, do so. Repeat on the next floor and you should hopefully be able to get your third. If the third one kills you, this will still unlock.

Enjoy having the easiest character in the game