You must have skipped every side quest. I was at level 28 when I reached Marleybone.
Yes, you're far too weak for Mooshu, and you're going to have the same problem for the rest of the game if you don't correct it.
Like it or not, you'll need to go back, starting in Wizard City, and run through all the areas, looking for side quests. Depending on your school, some side quests are necessary to get certain school spells. The experience you get from the side quests will give you the levels needed for more health and mana, better spells, and better equipment, as well as gold that's needed for the equipment, treasure cards, etc.
It would probably be a good idea that you find some sidequests you missed and level up a bit to get next spell or two. With that said, if you are already in Mooshu at that level feel free to try it out, since you finished the storyline quest you deserve to be there, but may have a hard go. Good luck.
Your too weak and esily be killed.. But.. I have a solution.. Go to WC, KT, and MB to look for forgoten quests.. Do all of them.. And try to lvl up to Lvl 35 to begin.. When your at lvl 35, you'll be perfect...
FYI, the first boss you fight, is Storm with 1,800 Health or 1,400 Health.. Make sure you got tons of Myth cards, and treasure cards...
Wow I am waiting for Mooshoo lol I am almost level 35!You should really level some more and get some more health but if you need help I am there to help peace Fiona FairyBreath fire.