Tate no Yuusha no Nariagari Season 2 episode 4
The Spirit Tortoise’s head was cut off, but Ost, its familiar, is still alive. It could revive at any moment. The allied armies need to get back into fighting shape, and fast. Naofumi makes the risky move of heading onto the Tortoise’s back with only a small group, and tries to find its true weak spot.
It can be here too: