The Binding of Isaac: Repentance

This achievement requires you to unlock endings 1-15. Ending 16 (defeating Mega Satan) is not required.
Endings 1-11 require you to defeat Mom's Heart 11 times in the Womb 2 or Utero 2 which is unlocked by defeating Mom once.
Ending 12 requires you to defeat Satan in Sheol.
Ending 13 requires you to defeat Isaac in the Cathedral.
(You unlock Sheol and the Cathedral by beating Mom's Heart 10 times but can go to Sheol with a deal with the devil or the Cathedral on a deal with the Angel by either going down a floor or going into the light respectively.)
Ending 14 requires you to defeat Blue Baby on the chest.
Ending 15 requires you to defeat The Lamb on the Dark Room.
(The Chest and the Dark Room require you to have defeated the Cathedral or Sheol 5 times each respectively and to have taken the Polaroid or the Negative to get there respectively.)

If you would like to do this easily or faster than natural play, I would recommend unlocking Azazel by taking 3 deals with the devil in the same run (can be 3 on 1 floor and the 3rd deal can kill you and it will still unlock) as he is the easiest character in the entire game.

Upon unlocking your 15th ending you will unlock Golden God.