Did you do the wysteria? That gives more xp than grizzleheim. I would just make sure you're lvl 38 and have triton before you fight the jade oni. If you are still worried, get some friends because it's a long battle with 7,500 health. If you want to solo, log out if you're second and go back in until you are first. Supercharge helps a lot when you're trying to solo big bosses like jade oni.
I almost finished wisteria and it gives you a lot of xp. But grizzleheim has more quests and more bosses. also, wisteria has two main bosses: lord Bramble life about 1800 life ( tangle wood way) and goblin king: life is about 2000. But overall, wisteria is really fun and easy