On the first encounter, take the sheltered path on the left. Let Triss, Vernon and the King run in first and just help them taking down the three enemies there. As soon as they're down, run to the next shelter. Use the Quen sign to protect you from the fire. Also make sure that you don't run into the open when the dragon is close by (it's circling above). As soon as the next three opponents are finished, run out of the shelter and take on the enemies by the gate. Move and roll a lot but try to not move out into the open.
The next encounter is on an open bridge where you have to protect the King. It's a quicktime event and you have to push the left (?) mouse button as soon as it flashes on screen. I've read that it's possible to turn these quicktime events off but haven't checked it yet. It isn't so hard if you know what to do though.
Basically, the first part is a video sequence where you just have to click the mouse button at the right time. Click it continuously right from the start and you can't miss the right moment. The next part you have to move Gerald, who's holding the King, across the bridge using the movement keys. As soon as you approach one of the corners, push the mouse button again to take shelter from the breath of fire of the dragon. When you've crossed the bridge there is one last quicktime event to finish the dragon off... well, to get rid of it at least.
When I first played the game this part gave me some headache as well. But it isn't so hard any more if one knows what to do. I started the game again yesterday and found the prologue relatively easy to play (on normal setting).
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