Fukushuu no Taishi - Level 9999 no Bakumatsu Isekai Tensei

During the end of the Edo Period, the Shinsengumi are suddenly transferred to another world. As they arrive, a guide from the other world, Hangya, appears and tells them, “There’s only one way for you all to return to your former world. Please, defeat the Demon Lord.” This is a tale of the weakest warriors becoming the strongest.