If you still have the Delkfutt Key from Rank 4, you can take a shortcut up from Lower Delkfutt Tower. When you enter Lower Delkfutt Tower, take a left at the first intersection and follow the right side of the wall past the stairwell and into a corridor. There will be a cermet door on the right, trade your key to it and it will allow you to enter (you do not lose the key). Go through the corridor until you enter a large room, take the first corridor on the left.
To the right of a long stairwell is the ??? at the bottom which will automatically transfer you to the elevator on the 10th floor of Upper Delkfutt Tower.
From now, if you went as THF (main), you can just find the chest and use thief's tools on it. You can avoid the key farming step this way. (Just the same way you can avoid on old gauntlets on AF quests)
Kill Gigas on the 10th floor to get a Delkfutt Chest Key. Elementals and Ghosts on the 7th through 10th floor also drop the key.