Deep in the mountain's caves is a lake wherein a killer whale resides. This whale is Dark Matter's enforcer in Ripple Field, guarding its dark influence from heroic puffballs. You will have to best it in battle, both on land and in the water!
At the start of the battle, this killer whale is fighting out of its element. Its only attack is to charge at you across the screen, dislodging a rock when it rams into the walls. Your strategy here is very simple. Just jump over it as it charges toward you and attack it as much as you can while it is reeling from the impact. If you lack a copy ability, the rocks that appear whenever it hits a wall can be inhaled and spit back at it to inflict damage. Overall, the main threat will probably be the small size of the arena, which doesn't give you a lot of space to dodge Acro's large body.
Once Acro's health drops below half, it will break the ground and take the battle into the water. The battle here is a bit tougher, since you have to fight Acro while swimming, where a lot of copy abilities are weakened, or where you only get a weak blast of bubbles if you lack a copy ability. Acro will spit out a variety of projectiles as it swims across the lake. Anchors sink down to the bottom, mini-Acros home in on you while skulls travel in a straight line. During this stage, you can use any attack to knock the mini-Acros or skulls back at Acro to deal damage. Regularly, Acro will accidentally ram into the sides of the lake, causing rocks to rain down on you. Once again, you can attack the rocks to launch them at Acro, dealing damage to it. After a few more reflected projectiles, this clumsy whale will be knocked out!
With Acro out of the picture, Dark Matter's grasp on Ripple Field is lifted. As its evil tendrils retract from the power of the Heart Stars, a new area has opened up. The arid desert of Sand Canyon lies ahead. Your next challenge will be to cross its rocky terrain, complete with dangerous enemies and mysterious landmarks.