Update: Game had a patch on the 11/11/2015. All achievements should now be unlocking as intended.
This achievement is unlocked by beating the Depths 2 for the first time. Mom isn't a particularly hard fight, but there are a few things you can do to make it easier:
1. If you come across it, the bible is an instant win item against this boss, making the fight very easy.
2. Watch out for the shadow of mom's foot. Her stomp does a lot of damage but leaves a big opening to getting some damage in yourself.
3. Keep away from the doors in the arena, Mom can attack you through them and there is very little chance to dodge the attack.
4. Keep the enemies clear as they spawn. Getting overwhelmed will cause you to make mistakes. Ontop of this, some of your attacks will pass by to hit Mom as she spawns in enemies. That said, there seems to be a limit to the amount of enemies that can be spawned at once. If mom happens to spawn in enemies that avoid you in the corner, leave them.
As long as you take your time and don't get careless, Mom is a fairly easy fight. Once the achievement is fixed, beating her on a new save will unlock it. No idea if it will retroactively unlock for those of us with it already done however.