If you're determined to finish, find/purchase or craft some Bolts of Mass Destruction, find the biggest blank spot in the mini map you can find, then try and blast your way inside. BoMD destroy roughly 6-9 tiles of wall if aimed properly and can punch your way through to the secret room, if indeed it's there.
with many attempts at using the spatial instability potion, i finally managed to get in a narrow single hex wide corridor inside some walls. And 'lo and behold- there he was. I thought Dredmor was immune to stuns. I just abused nerve staple spell and he couldnt get a move in.
Are you on an older version? I thought 1.06 had fixed his stun vulnerability. Well either way 1.08 did. Congrats though
