The Sludge Goolix is a boss in Pikmin 3:Return to the Pikmin Planet. It is only found on the only and final sublevel of the Stone Labyrinth in the Desert of Wrath area. This boss is not very hard, but it is very LONG. It looks like a Black Goolix that is much bigger.
The nucleus inside the enemy will glow one of any Pikmin color. Swarm it with that color and that color only= Any other color will drown, even if it's blue. This will not do much damage, but it's the only way to attack. After a couple seconds it will change colors and the other colors will fly out. It is possible to defeat with only one color- You just have to wait until it changes to a color you have again.