His special ability allows him to become completely invulnerable to any plant attack for a short period of time every few second.
He can be fought in Mount Steep; specifically in Steep Mines, near the drill; and defeating him unlocks the ELUSIVE FOE: Dig Wig medal.
Dig Wig is found near the giant drill in the Steep Mines of Mount Steep. His ability to becomes invincible can be a bit annoying. A good strategy is to play as the Chomper and use the Burrow to swallow him, as if he becomes invincible while he is in your mouth, you will still be able to swallow him.