It's recommended that you bring multiple Bounty Hunters or Hellions to a fight, since they can easily hit the backline and deal a large amount of damage. This means that the Leper and Crusader are generally useless in a fight against the Hag.

First off, to counter that she is in the backline is simple enough: don't bring anyone with you to fight the Hag that can only hit the front two slots of the enemy team. To counteract these is simple in theory, but sometimes will not work out the way you expect it to be. After this they will pop out with a Death's Door status and be moved to the front of the slot, which could also affect your team's efficiency as shifts will almost always occur in positions. They will continuously take damage until the pot is knocked over by dealing enough damage to it, or they hit 0 health. She will always use this move when there is nobody in the pot currently, and she will usually be the first one to move at the beginning of a round. She's more difficult than she looks, mainly for two reasons: she resides in the backline of the team, where most frontliners have a difficult time hitting, and one of her moves, called 'Into the Pot', will remove one of your characters from the game and put them into a pot 'enemy' that takes up the frontline of the team (this pot cannot be destroyed by usual means and does not require death to complete the boss, simply killing the Hag will kill the pot).

The Hag boss resides in the Weald dungeon.

Darkest dungeon wizened hag strategy