All four hunters are locked in separate rooms. Cycle through all the characters until you have all their circular chambers cleared. The prisons represent the PlayStation 2 controller. Whoever is at the topmost cell is Triangle, at the right will be Circle, the bottom is X, and the left will be Square. The picture designs on the wall represent the shapes of the buttons. Move the hunters onto their corresponding buttons. The crossbow is to throw you off; ignore it.
Switch to Sundano and force every other hunter not to attack. Do not attack Frankenstein. Make sure your MP bar is high (attack some of the experiments to raise it if needed) and back away from Frankenstein. If no one is provoking him, his arms will slump and he will raise an electrical generator over his head and eventually throw it. While it is still over his head, he is completely vulnerable to attack. If you can see his shoulders slumping, immediately summon the Forest Spirit and the lion should eventually attack the vulnerable monster, knocking him down and dealing great amounts of damage. Followed with some combo attacks, you will severely deplete his HP. Before he gets up, back off quickly or else he will attack you. Wait until his shoulders slump and repeat the summoning and the combos. He should die quickly.
Using Jeager (the scientist), dodge the water balls that the Sea Creature shoots and wait for it to leap out of its puddle. Quickly run over and press Square, electrocuting him. He will be stunned for a little while, during which time you can use combo attacks to beat him up. He will return to the puddle. Wait, dodge, and repeat again until he is defeated.