Unfortunately, in addition to Hantengu being weak compared to some of the other demons, Mitsuri was the least effective of the Hashira in the final fight against Muzan, and she was notably the first one forced to leave the fight to recover their strength. Despite these caveats though, Mitsuri...
Knowing that she lacked the physical strength to beat someone like Doma (the second-strongest demon surviving under Muzan Kibutsuji), Shinobu allowed herself to be eaten by Doma to weaken him and allow a killing blow. This particular poison was so powerful that it actually melted Doma's flesh...