Is It Wrong to Try to Pick Up Girls in a Dungeon? On the Side: Sword Oratoria, Vol. 17 (manga)

Looking for the key to the man-made dungeon Knossos, Bete ends up going on
a date with the Amazon Lena, who has a one-sided crush on him. While he succeeds
in getting information from Lena, as the two of them look for the key, they are
set upon by a group of assassins! Their fiercest enemy, Valleta, stands in their
way, and she's not going down without a fight!