The most challenging campaign.

The thing is, if you don't stand in his way, and position yourself so that he can't go around you, he'll just rush past you, hit Nicholas, and that's it – "Mission Completed." By the way, it's not at all necessary to TAKE a garrison for this (it's a matter of taste); you can stand right next to him. I took a garrison just because I like large-scale battles (oh, what a bad person came up with the level limit in campaigns; he was wrong:( )