Yes, you do have to go through a tower to get your level 48 spell. I am not sure what creatures you have to defeat, since it has been awhile since I have been in that dungeon. Myth does sound about right. You will have an opportunity to adjust your quest before you go into battle, so if they are myth, just make sure you have your conversion prisms and you will be fine.
the tower has three levels and if my memory is right it has only one myth and he is the last one there and when i was there i went out side and got health after every floor
You can't leave without starting over. It's a dungeon. If your second school isn't life, just stack up on life healing T-cards or let yourself get defeated. Go heal yourself and comeback. If I remember correctly, you don't have to start over if you get defeated.
