I just killed the tree and went from a level 30 zenmaster to a level 79 zenmaster. I am wondering if it is possible to become imune to all magic. Can I harm the Jinn? Why am I leveling up so quickly now? I jumped to level 85 after 20 minutes more of play. Is it so high that it won't double any more?

you get roughly 10mil xp for the tree, if you wanted more levels like that, you could reset adventures, and go back killing him again for another 40 levels... it costs 240k xp to get a level after level 14 i believe, and that number wont change unless you take another class, once that class reaches 14 then its back to 240k per level...