I know what you mean. You have to shoot the dragon head with a STRONG flipendo spell, this will make it go up and give you enough time to run back and to the left side of him. Then, you have to cast a spell at the torch that is just to the right of the dragon head, spin round and light the torch that was behind you, but which you are now facing. Run forwards, past the stairs that lead to a barred door, past the next torch and then light the torch facing you, then spin round and light the torch that was behind you, but which you are now facing. Then you should hear a creaking sound, which means that the bars that were across the door that you need to go out of has been raised. Run back to the stairs that you had just passed and up and through the door and you are out! This may take you a bit of practice, as you have to move fast and you must get all the torches lit before the bars will raise. If you don't hear the creaking sound when you have lit the last torch, that means that one of the has gone out and you need to start again.