The Binding of Isaac: Rebirth
637 Achievements
800-800h
Isaac's Heart
Unlocked a new item. Details for this achievement will be revealed once unlocked
19.5%
How to unlock the Isaac's Heart achievement in The Binding of Isaac: Rebirth - Definitive Guide
Everytime you back out to the main menu you get an autosave. So keep that in mind.
After you've cleared one or more (how many you like) rooms without being damaged exit to the main menu to get another autosave.
In case you get hit you have to either let enemies, spikes or bonfires kill you or bomb yourself to death. This can be a tad annoying if you have 12 hearts towards the last couple of floors like the "Womb/Utero" or the Darkroom/Chest.
NOW THE IMPORTANT PART!!!
During the death animation pause the game and hit the guide button. Manually quit the game.
VERY IMPORTANT!!!
You MUSTN'T quit after being hit BEFORE you die. That way it would for whatever reason save your current health.
VERY IMPORTANT!!!
If you continue after starting the game up again you will be back where your last autosave was.
This works for THE LOST playthroughs as well!!!
Of course with him killing yourself is easier since he has no health and dies in one hit.
Items like Holy Mantle (you can take one hit every room), the "9 lives cat head" or guppy's collar (1 extra life) can change this, though.
Avoid the Ankh at all cost since it respawns you as another character and would therefor void the achievement.
So just pray to the RNG God to be lucky with drops and this is going to be a breeze. Time consuming but easy.
Good luck everyone!
Edit 1 (Credit for Slam Shot Sam):
Worth noting that if like me you can't be bothered going for the platinum god achievements, but still want the Lost achievements, you can play on medium to make it a little easier and also get both achievements in one run by exploiting saves.
Exit game just before the It Lives fight to make a save and then go offline. Beat either Satan or Isaac offline, then go to My Games & Apps and delete your save data. Go back online and the achievement will pop, then load up the game and it will sync back to your old save. Then it's as simple as killing It Lives again and going to fight the opposite boss for the other achievement.
If you vote negatively please state why in the comments so I can improve my solution. I know some people prefer going for this legtit but this is for gamers like me that don't have enough skill to attempt it that way.
Exit game just before the It Lives fight to make a save and then go offline. Beat either Satan or Isaac offline, then go to My Games & Apps and delete your save data. Go back online and the achievement will pop, then load up the game and it will sync back to your old save. Then it's as simple as killing It Lives again and going to fight the opposite boss for the other achievement.
On the plus side The Lost starts with Holy Mantle after you've donated 879 coins to the Greed Machine and Afterbirth adds a whole load more items including a bunch of very powerful synergies which can be used to make the game MUCH easier.
That being said you may still be better off getting your 6 red markers with The Lost before buying Afterbirth, that save exploit is a lifesaver.
I found a very good article on Destructoid with detailed tips for playing as The Lost which may be useful to people: https://www.destructoid.com/binding-of-isaac-10-tips-for-mak...
The Lost is the hardest character in the game to play as because you die in one hit and can't increase health by any means. You either have to be extremely skilled to finish runs with him or get lucky and get high DMG and really broken items like Holy Mantle or Stop Watch.
Currently, there is a glitch that can be performed to negate a game over when you die. Press and exit the game before entering a room to create a checkpoint. Continue playing and right before you are about to die, hit the guide button before the game over screen appears, quit the game completely and when resuming you will continue in the room before you died. This makes runs with The Lost a joke until it gets patched so feel free to do this if you are really having trouble.