Really, that was his whole plan? After so many years in the world, he's developed zero combat capabilities yet thought that he could somehow do it himself just by using potions? Also, even having played the game before, he somehow didn't realize that just that wouldn't be enough? And also he decides to bring along a supposedly normal citizen into a highly dangerous dungeon just cuz she said please? Also his whole party would be doomed were it not for this supposed normal girl just happening to have what they need to get themselves out of this mess?
I guess this is going to be yet another one of those stories where the main character is a halfwit who needs to rely on luck, happenstance and just plain old plot armor to get by.