Use your solitaire skills to assemble powerful combos and defeat the dungeon's monsters. Grab treasure and spend it to build a set of powerful, synergistic abilities to help you. Manage both your own resources and the demon's patience to hopefully survive the ordeal.
You are an adventurer who entered into a pact with a demon. That comes with wealth, power and other usual perks, but the contract does stipulate a teeny, tiny favor: could you maybe clear this slightly monster-infested dungeon first? The demon is kind of sealed inside and it would really appreciate you punching a hole in its supposedly-eternal jail. That's definitely a safe thing to do with no far-reaching consequences whatsoever.