Will - O - whisp are ghost pigs that appear in star reef. They attack by their bright lamps. Their passive ability is very annoying ethereal in which they apply it to any single pig including themselves which have 50% chance to dodge attack. They are summoned by banshee too. Not only star reef they even appear in bamboo forest , snow peak where they are summoned by banshee and old nesting barrows 20% if the levels are replayed.He also appears in hog mountain where he is accompanied by either spirit caller or pyro pigs.
STRATEGY
Since they use their annoying ability to a random pig it might become impossible to beat that pig or it only. So defeat it first so he may not apply that annoing effect which protects other pigs. You can remove the spell by classes.