Don't bother with the spells and crystals stuff. Do XP first then the attack, defense, speed, agility. Rush the other team every lvl right away. Nothing can stop you.
Giant and Samurai are great but not needed for the last island. I used random crap to beat the last Island. All you have to do is attack from an Island and retreat after killing 10 guys or so. If you do that from 2 Islands before they use any kind of res. spell by the time you hit them from the third Island all they have left is the boss. At that poit you can beat it with just about and unit.