Ruckus is the boss of Level 20 in Bubble Bobble Part 2.
Grab the lightning potion. You can't hurt Ruckus without it. Ruckus is faster than you. Ruckus attacks for the most part by running into you. I recommend running in front of him, and spitting bubbles and running into them. They'll shoot back and hit Ruckus. Ruckus does have an attack where he spits a bouncing red ball at you. If he pauses, jump up FAST. He doesn't use it often, though, so it shouldn't be a problem. I also wouldn't worry about trying to run ahead of him the whole time, either. You can't hit him from behind, but it'shard to stay ahead forever. Just attack over one or two plank's worth of chasing and when you have a chance jump up. He'll continue his path to the bottom of the screen and start back up at the top. This is a very easy fight. After 15 hits, you'll be done with Ruckus and the first zone.