For me it was probably the plague oni since he is balance and can cast weaknesses and lots of shields. I also find youkai in the tree of life (secret boss) . he has as much health as malistaire but drops good loot. The jade oni was a bit hard but not that bad.
I'm fire, so the War Oni was a pain, and the Plague Oni was hard ( cause he's balance) but overall, the hardest boss I've fought in MooShu so far is Takeda Kanryu. You have to defeat him for the smoke screen spell. Even though he has only 1300 health, his immunity to fire is way too high, and he has two minions! His Otomo soldiers are ice, 675 health, but use a balance deck instead! I couldn't defeat him until someone else came to help.