Mixed Martial Arts (MMA) is the ostensible subject of Ngozi Anyanwu’s taut two-hander The Monsters, now running Off-Broadway at Manhattan Theater Club after a fall stint at Two River Theater in New Jersey—but the play’s true combat is emotional. Beyond the excellent choreography and fight direction (by Rickey Tripp and Gerry Rodriguez, respectively), the deeper exhilaration stems from seeing two actors, Aigner Mizzelle as Lil and Okieriete Onaodowan as her big brother Big, deliver beautifully realized performances.