Working Seems Pointless After The Hero Labeled Me As A Coward And Kicked Me From His Party summary:

A martial artist called Nicola, who specializes in techniques considered foul by many, is kicked from the Hero's party. Feeling betrayed, he vows to never work again. Nonetheless, Nicola's older sister, Satella, has other plans for him, appointing Nicola as the teacher of the class composed of the weakest alumni in the Shanoa Academy. There, he meets Alice, a tender and lovely elf girl. In a world where a person's worth is determined by their muscles, Alice ranks among the lowest, and to change herself, she enlists Nicola's aid to learn from him a fighting style in which... anything goes! This is a battle fantasy story of rising to the top through clever but underhanded tactics!