The toughest part of orchestration thru midi is the realism, ime. Accents, glides, trills.. these are tough to recreate naturally without some nicer orchestra software.
Otherwise, composing for an orchestra is a whole new language in music, if you're not familiar with it. Depending how deep you wanna go (and the WD soundtrack goes deep), you're not just talking about the trumpet section.. but often there are 1st, 2nd, and 3rd trumpet parts within the full group. Same goes for woodwinds... alto sax is used differently than tenor. Oboe vs Clarinet? Piccolo vs Flute? Viola, Violin, Cello, DoubleBass in the strings section.. and they also typically have 1st and 2nd (harmony) parts within each instrument group.
Like I said, if you wanna get heavy into it, you can learn a lot about music studying orchestration/cinematic music. Check YouTube, or just listen to some composed music. Some online lessons will have you listen while they scroll the music in front of you. Even if you can't read music, you can follow along enough to realize how everything is working together.