Fukushuu no Taishi - Level 9999 no Bakumatsu Isekai Tensei

It is the end of the Edo period. One day, the Shinsengumi are suddenly reincarnated into another world. Hangya, their guide to the other world, tells them, "There is only one thing you need to do to return to your world. Defeat the Demon King." This is the story of how the weakest soldier becomes the strongest.