So I just killed the Shadow Yukiko boss. This was my second attempt as my first was one of those "I've saved so I'll just mess around til I die" runs. I leveled up to lvl. 18 and fused myself a Jack Frost and she became almost insultingly easy. Anyway, I had my Ibanez on my lap whilst playing and between turns I had a plinkle...then stumbled into the guitar melody for the Boss Music. It's thus:

e|--------------------6-----------------------------
B|------8---6------8---6------8---6--4------4--4-
G|5--7---7---5--7-------5--7---7------7--7------
D|--------------------------------------------------
A|---------------------------------------------------
E|---------------------------------------------------

Hopefully that has turned out ok. Anyway, from there the melody is the same and you can figure out the rest as you go as it stays the same shape. I'll probably tab it fully sometime, but I don't have an uploading files account of sorts anywhere.