I am a life wizard, which made the battle longer, but not necessarily harder. I had to use a lot of life to death prisms and primarily used my secondary school (fire) attacks. However, I found that he was an enjoyable battle, even though it was long. Focus on your deck. Make sure to bring shields, blades, and traps, and you will be fine!
I'm a Pyromancer so it was pretty easy... first I loaded up traps, blades, put on Wyldfire, and then used a Heck Hound on him, (it did over 1000 damage per turn) and proceeded to randomly attack him until he died. My minion (Fire Elemental) helped a lot because it can spam fire elves and links at enemies. So yeah... I soloed him and it took like 20 minutes. ( PS, I think the Oni defeat animation is awesome).
I found him quite easy actually, I am a life wizard but secondary storm so I just sent out all my storm spells. Although I forgot to refill my mana before the fight so I ran out when he had like 500 health left. But luckily I had a friend online so I asked him to come finish it off :D
It really depends on the school. If you are any school other than life, I'm pretty sure it's easy. However, if your a life wizard with only life spells, it's pretty difficult to defeat the Jade Oni. I did mange to defeat him solo, but it wasn't easy because he kept on casting death shields on himself.