Waterwraith

The Waterwraith is a boss creature from Pikmin 2 that has no physical form in the Pikmin dimension. In the story mode, it is only found in the Submerged Castle area, which you can only bring Blue Pikmin into, although it is possible to find Bulbmin on certain floors. It rolls around using it's two massive rollers and tracks down Olimar/Louie/the President and their Pikmin to crush them. It's only weakness is the strength of the Purple Pikmin, which can anchor it into Olimar's dimension.

Strategy
While traveling through the floors of the Submerged Castle, you must clear the first floors quickly, because after a period of time, the Waterwraith will fall form the ceiling with it's rollers and automatically chase after your active captain, no matter where he is. On these first four floors, it is useless to attack the Waterwraith, as it is physically present in another dimension and thus there is nothing to attack.

However, on the fifth and final floor, you are given two Violet Candypop Buds, which you can use to turn ten Pikmin/Bulbmin into Purple Pikmin. Use nectar from the nearby eggs to flower your Purple Pikmin. Break down the gate leading to a large room, walk to the center, and the Waterwraith will fall down once again. However, throwing Purple Pikmin next to the Waterwraith will anchor it in this dimension, allowing you to attack it at last, though it will try to shake Pikmin off and unanchor itself after a time. Repeat this over and over, and eventually it's rollers will break and it will run around the floor in panic, harmless. Continue pounding it with Purple Pikmin, eventually killing it, and giving you the Professional Noisemaker treasure, which allows you to unroot Pikmin in the ground with your whistle alone

Piklopedia Notes
In the Pikmin 2, Captain Olimar and Louie keep records of every creature they have faced. While Olimar lists scientific facts, Louie normally only lists recipes on how to eat a creature.

Olimar
All that is known about this creature stems from a few sightings deep underground. All reported sightings feature the same core set of details: a giant, viscous form with a clear, hazy sheen not unlike hard candy. One theory holds that it may be the ectoplasmic incarnation of a kind of psychic phenomenon, but as is usually the case with such theories, it is very difficult to prove. All witnesses report being suddenly overcome with fear upon sighting the creature, approaching a state of panic and near insanity. In fact, every report contains an inordinate amount of extremely vague details, which has led to suspicions that exhaustion and fear have caused some simple natural phenomenon to be viewed as a living creature.

Louie
Inedible. Known to cause mass hysteria, followed by leg spasms and internal thunderings.