The first boss after acquiring the Bear transformation power is the giant toad boss in Arareko. You have to use the bear transformation to knock rotten fish into the frog's mouth when it is inhaling to spit at you. This causes the frog to fall down with its tongue out, which is the only part of the toad you can damage. I used the Bear transformation attack when the boss had 83 life left to kill it. It was the only time I used the attack on the boss so I am unsure how much damage it does. Remember that you need a full magic bar to use this attack.