I went through the Hole of Heroes and was able to get all but one treasure. The one treasure was held by a Man-at-Legs, which was a pain in the neck to fight because there was so much water on the field. I was forced to skip it and keep going.

The final boss, the Raging Long Legs, was easier and more fun to beat. After I beat it and took its treasure, I returned the next day (Pikmin time) with 100 blue Pikmin and returned to the floor with the Man-at-Legs. Even then it was hard to beat, but EVENTUALLY, after many soft-resets, I was able to beat it and get that last treasure!

Now I eagerly (and impatiently) await Pikmin 3 for the Wii U.