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!