It’s you versus Kaos! Beating Kaos is no mean feat. You have to work on your timing to survive this encounter, because you won’t get tons of buffs, healing, or support here.
Kaos’ first attack is to teleport into one of the upper corners; he shoots a few waves of spiked cubes across the screen. Jump over these as you make your approach, and then counterattack as aggressively as you can before he disappears.
The next stage comes after Kaos has taken some damage. He teleports again but appears near the center of the screen. Also, there are several Kaos (Kaoses? Kaosi? Whatever they are). However, you can easily identify the real Kaos, as he has a crown. Area of effect attacks let you thin their ranks quickly, because the fake Kaoses disappear when they take any substantial damage. Find the real one quickly and hammer on him.
Kaos gets nervous after he’s lost about a third of his health. He pulls back and tries to use a blade of darkness against you. Don’t try to hurt it; play defensively and wait out this attack. Don’t stand in the marked blue area; that is where the sword is about to strike. Pull back from there and then jump over the shockwave it creates. Do this for a few rounds and eventually Kaos returns.
He starts to mix up his teleports, cloning, and sword attacks. You have to really be on the ball to dodge everything, but it is quite possible. Keep your health high with a defensive fighting style, because you can’t simply burn through Kaos’ health. He has way too much of it. Be patient!
When none of these attacks manage to destroy you, Kaos summons two pink hands to bash you and create large shockwaves around the area. Leap over the waves, and strike back when the hands fall to the ground in between series of attacks. Stay as close to the front as you’re comfortable standing, because this gives you more time to counterattack the hands. It’s not worth taking hits to do this, but if you’re good at dodging the shockwaves it’s a solid way to improve your damage output.
Late in the fight, Kaos gets to spawn even more clones when he teleports, and he starts to blast the screen with many attacks. He also becomes invulnerable for brief periods. Take it all in stride; he is an emperor of evil, after all.
Stick to this long enough and Kaos gives up the fight. You’re the best. You know it! And now everyone knows it. Time to pack up, do some touring, sign people’s books. It’s all good now.