So I looked online how to mine dungeon brick and I read that you need a DEATHBRINGER picaxe and you get the material by beating the brain of CTHULU and I did!!!! So I have all the material I need to craft it and I might be able to get in the dungeon without getting recked by the dungeon guardian!!!!! (P.S. this is for ios and I think for other systems I'm not sure but I hope it works!!!)
I just got the DEATHBRINGER picaxe and it did mine the dungeon block but the moment you drop down into the actual dungeon you get killed by 4 Skeleton heads. But I think I can beat him now beacuse I made a super powerful sword.