You equip two shields on all your characters, then you use magic to kill all but one enemy, which can't escape, then keep using attack and you won't actually attack, you will just keep blocking. Then after a while kill the enemy and your shield level will go up, but if you change weapons in battle, so like your two shields for two swords your sword level will go up. The easiest way is to just hold A for a while, so if you have blutack or a rubber band it works well.