Ride next to Stygian, every now and then he will just ride along the surface giving you the opportunity to use your sword and knock the metal guard over his face off. Once you get it off he will start coming after you like the first worm you faced, so just shoot that little yellow thing on his mouth. After awhile he will start following you underground. He will pop up straight out of the ground trying to ht you, just turn tight and use boost. The other thing he will do is send smaller worms after you, about 3 or 4 at a time. They are quite fast so turn sharp and just keep shooting them. After a few rounds of doing this you will get the chance to run up to him and hit B for the kill.