Wicked Wurm is the boss of stage 4 from Battletoads (Game Boy).
The Wicked Wurm is a large aquatic worm monster. It shows up at the end of the stage's obstacle course, approaching from the right but not really attacking. At this point you can use your jetski to attack, so all you need to do is approach the Wurm and whack him repeatedly.
After taking enough damage the Wurm's head will detach and start flying, moving around in an attempt to hit the player. You just need to time it as the wurm descends to attack it and the boss will be gone after some hits.