If you are there on a night when there are two showings of Fantasmic the second show is much less crowded. For our last two trips, each night Fantasmic was showing, it showed twice. Last January the second showing was to an almost empty crowd. In May it didn't fill up either and that was the week before Memorial Day.
Avoid the area near the amphitheater entrance during the show switchover! Between people exiting, people trying to get to Tower of Terror, etc, it is a zoo. But by about 15 minutes before show time, the first show crowd has cleared out and those who lined up for the second show have been allowed into the stadium. Then you should be able to just walk in. If there are no seats, you haven't lost anything but the time it takes to walk up and check. You probably won't get front and center, but there might still be seats depending on what time of year you go.
And you get to enjoy the park while about a zillion people are watching the first Fantasmic so the lines are pretty short for most things.