you first see three chains, right? well, first, quickly jump on it. then, swing from chain to chain. avoid the spiky tail, and there is a switch highlited in white. you click it, and the dragon goes to sleep, for only a few seconds, so you have to move fast. once you click that switch, go in front of the dragon, and use your ice arrow and shoot it into the dragon's mouth. repeat 2 more times.
you need to defeat the dragon
No. (see related question)
no there is not a cheat to get past the dragon on astro knights
You don't defeat knights, you talk to them and they'll give you a weapon to defeat a beast. (On the jungle planet, find the Laser Lance down in the plants before you meet Sir Cador.)
You must defeat all three planets (tigercopter, phoenix birds, and dragon) to advance to the end of the game. When all three knights are accompanying you, you can find the Princess using the Golden Key on the secret giant asteroid. (see related question)
Sir Cador joins you in your spaceship and you fly to one of the other two planets you have to defeat, the ice planet and the fire planet.
Sir Peleas is deep within the volcano, at the lair of the Fearsome Fire Beast (a robotic dragon). He will give you the Ice Arrow to defeat it. (see related question)
If you have already defeated the dragon then there is nothing you can do, but if you didn't defeat it then see if the person who created poptropica has a e-mail address and email him what happened.
on astro-knights.
You get the ice arrow from him to defeat the Fire Beast (mechanical dragon). Once you have beaten all three planets, the knights will tell you of a giant asteroid. It is at X-11 Y-82. Only go there if you have all three knights with you. (see related question)
Astro Knights
The Laser Lance is down in the metal flowers on the Jungle Planet. (see related question)