In The Witcher 2, is it possible to hit the man with the red plume?

According to the Game Guide that came with my copy of the game, 1.5 degrees is the right angle. I've got my brother testing what that does currently, and will update when he succeeds (or fails).

Edit: It appears that if you get the correct angle and kill the "red-plumed man" you will not have to fight Count Etchverry after breaking through the barricade. (so it reduces the number of enemies you have to fight a little later on).

I've found it much more effective to aim the centre of the crosshair at the middle of the wooden structure he's on; the mid-left crosshair should be on the left-hand guy.

As with most siege weaponry, the goal is to hit the structure, not the individual.

After trying 10 times to hit him, I just aimed for the entire thing and got it first go.

I'd also reccomend making a quicksave just before it (after Foltest talks to the arbalist) and hit reload whenever Geralt doesn't order "One and a half degrees".

Good luck, I found this to be a major hassle too!