The Binding of Isaac: Rebirth

Do not sweat this achievement, It is one of the easiest in the game.
If you have Bible, then use it as soon as you walk into the room to fight, Mom, Mom's Heart, or It Live's, all of them will instantly die.
And Bible is one of the items that you have to do nothing to unlock.
So keep playing, and eventual RNG will be kind and give it to you.

Note: Just keep it in mind the instakill you will get on two bosses. This can be very useful if running with a tough character (I.e ???, or The Lost) it is a lifesaver on those runs!

The Bible can spawn in a library, angel room, or shop but I've personally had more luck finding it in the shop. If you're not having any luck finding it, leveling up the shop to at least level 3 (200 donated coins) will help your odds seeing as 1-2 pedestal items can spawn instead of 1.

Keep in mind that the Bible is an activated item which requires 6 bars of charge. I wouldn't recommend using it when you reach Depths/Necropolis II until you fight Mom unless you find a battery.

When you reach Mom, use the Bible right away (LT) and it will instantly kill her.

The bible is an insta-kill against mom. Therefore all you have to do its obtain the bible as an active item and then save it for the mom boss fight and or the Mom's Heart boss. Once you have the bible don't die until one of these boss fights and as soon as mom is on screen slap LT and the achievement should pop.

This is just a tip, for those who doesnt know, if you use the bible against satan it will INSTANT kill you...i figured that out the hard way...