You need to look for a weapon.
It's hidden somewhere here on the face.
Use the loose tooth, and you'll find the toothpick hidden in Colossus's mouth.
But before you can get rid of the big flea, you'll need to take care of smaller ones.
Walk up to the scar through his beard slowly, and if you stand at the top of the scar for a few seconds, a baby flea should pop out.