Hi, thanks, glad you enjoyed it :-)
I think the requirement to kill the wraiths probably stems the fact that in the movie, they are all washed away in the flood of the Bruinen, and even in the books, 3 get washed away initially, and Aragorn, Glorfindel and the hobbits arrive carrying flaming brands and drive the other wraiths into the waters as well...
So I can see why there is the need to kill all the wraiths... to win the scenario
The only bit I was unsure about was the timing of releasing the flood and how to actually catch evil models by it???