If I remember correctly, if you load up Miraz's Castle Assault, three soldiers will come out of a door every few seconds. Keep hacking away at them with Caspian, and you can work toward the 200 kills you need with him. Also within the Castle Assault section is a portion where you are in a stable. I don't recall the name of the level, but keep replaying it and you kill a few enemies on horseback if you wish within that section. Keep replaying that level and you will get it...it is the portion where you must put a couple of weights onto some platforms and then open up further sections of the stable. I think you get about four enemies you can kill from horseback each time through.
I'm against picketing but I don't know how to show it.