this is a cut & paste excerpt from familyfun site re: best way to see fantasmic
In the busy season (i.e., summers and holidays), it is advised that you be at the Fantasmic stadium at least 90 minutes before showtime to guarantee a good seat. And you'll need to be there 60 minutes before showtime to guarantee yourself any sort of seat at all. In the off-season, 60 minutes is usually enough. Since this is admittedly a long time to wait, some families leave one family member (along with strategically placed jackets and fanny packs) to guard the turf while the others visit a final attraction or shop. You can also eat in the stadium, so if you're pressed for time you can either buy snacks from the stadium vendors or carry in dinner from another fast-food place within the park. A clever comedy preshow helps to make the wait bearable.
The best seating is as close to the center as possible. The sections at the extreme right or left of the stadium miss a bit of the live action, although the water screens are situated to ensure good viewing for everyone in the stadium. But don't think that a front-row seat is any particular prize. You'll get more of the scope of the show if you're sitting about halfway back.
When the show is over, there is a mass exodus toward the gates. The crowd is really something to contend with, since nearly everyone converges for Fantasmic! In essence, everyone in the park is trying to exit at once.
If you're sitting near the back of a central section you have a good shot at being one of the first ones to exit. Get up and get out fast. But if you're down near the front or near the sides, there is no way to make a quick exit. You're better off just sitting still and letting the bulk of the crowd leave before you. And since it is nearly impossible to push a stroller through the crowd, families with very young children will definitely do better by waiting until the area is at least halfway empty before they attempt to exit.