Go from Leshan to Chengdu airport for an 8pm flight so train no good (too early or too late).

The schedules of the bullet train are 07:39, 09:36, 11:06, 12:41, 14:29 and 21:05. It takes no more than an hour on the way. No suitable choice for you.
So you are suggested to take direct bus from Central Bus Station. The timetable is 07:40, 10:00, 10:35, 12:40, 14:00 and 16:00. It costs CNY 46 per person. The duration is about 2 hours. You can choose the bus scheduled at 16:00.