plot (to make him show off) aside, isn't they incompatible?
the sensei swordmanship is something build to fight against human opponent
while the adventurer need to fight horde
es of monsters (or adapt on the fly) plus potential backstab from other adventurers, and also environment adaptation and supply management (if long term dungeon explorer)
which wouldn't be out of place if sensei got overwhelming advantage here, but can't measure much for his survival skill