In the process of speaking with a Formorian (the gray ram thing) in battle, you may end up being given a job of sorts. This job, "Capture the Berserker", requires you to kill Wu Kong, a boss you can fight on the 4th Stratum after accepting the job.
As a reward, you can get the Tokkosho weapon. When you do obtain it, immediately equip it - it's about twice as powerful as your start weapon, and 50% more powerful than the Dies Irae you hopefully have.
Since we're speaking of Wu Kong, let's detail him. (Keep in mind that battling him will be taxing. It may be a good idea to backtrack to the Barracks at points.)
Firstly, he'll appear on the 4th Stratum. After descending one ladder, head south and cross the bridge to find him. After that, he'll appear on the 2nd Stratum, in the alcove above the southeastern room where you fought a boss and got the Woman's Ring. Afterwards, you'll find him due north of the entrance of the 5th Stratum.
|20||Fury||Ice||Fire, Electric||Physical, Gun, Light, Dark|
During the first two fights, this guy shouldn't put up much of a fight - I beat him at Level 12. While it is interesting in that you pretty much have to have MP for this fight, with basic physicals being nullified, it's not nearly as bad as it seems. You can Bufu him to death, pretty much - he doesn't have a lot of HP.
The third fight is more difficult. While he does retain a lot of his old repertoire, he also likes to chain stuff together. For example, Crusher Wave will reduce you to one HP, then he can just sneeze at you to kill you. He'll actually do that kind of crap a lot, so finish him quick.
The offense remains the same as before, although none the funner.