Cuphead

Cuphead is a run-and-gun platformer from Studio MDHR, built around boss battles. It looks like a 1930s cartoon throughout: hand-drawn cel animation, watercolour backgrounds and an original jazz soundtrack. Cuphead and his brother Mugman lose a bet to the devil at a casino and now have to pay off the debt.

You travel through strange worlds, acquire new weapons, learn powerful super moves and hunt for secrets. Every boss has several phases, and attack patterns must be learned and dodged on time, so the game teaches you through failure. You can play alone or in local two-player co-op.

According to HowLongToBeat, the story takes about 11 hours and a 100% run about 28 hours. In a month at two hours a day (about 60 hours) you can beat the game with all its challenges; difficulty varies by player, so times differ. The game is single-player and fully available on rental.