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:( )
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