the dungeon is pretty easy, just equip your items before entering so you heal a lot more, then mostly just use the meditation one that heals several times over time. Also, you can change her path by walking in front of her, she will move to the side if the ai notices you are walking in front of them so you can use that to make her walk past enemies. The boss is a bit tricky but if you just pop some mana pots it is doable, just skip the melee buff since afaik it does nothing, just cast meditation and if needed the regular heal. You get free pots from people outside of the dungeon who are standing next to the gravestones, but you can also just buy from the slime.
You don't get anything from beating the boss but you might in the future