Play as Isaac, or Samson (If you have Samson unlocked)
Refuse any devil deals you come across, and save all Eternal Hearts (They're white in color) until you find a second one, or you're done with the floor you're currently on.
Once you have 7 heart containers you'll have the achievement, and the character Maggie unlocked.
Alternatively, you can get 6, and use the Strength tarot card (This might work, but I'll need confirmation for it.)
EDIT: The Strength card trick
WILL work (Thanks
https://www.trueachievements.com/PHYCO+LLAMA.htm)