Level 1 Demon Lord and One Room Hero Vol. 5 (manga)

The Demon Lord is displeased with the cramped bathtub in Max's one-room apartment, prompting an excursion to the local super-spa! After Max's three-way battle against Fred the Cleric and Leo the Warrior, perhaps he too can finally get some rest and recreation that doesn't involve his futon and fast food. However, the demons are starting to awaken from their unholy slumbers in the underworld, and they crave the return of their master... so Max may just need to do his stretches, put on a clean shirt, and wreck some havoc!