Try filling the tub with at least 8 inches of water and plug up the air vent above the drain so the drain is air tight. Use a large plunger and lots of muscle and vigorously pump the drain out. A large plunger or one of those accordian style plungers work well.
On another occasion when the plunger trick didn't work I routed a garden hose through my bathroom window and forced water down into the drain to clear the blockage. Be sure to make a tight seal to force water pressure down the drain. I had to seal the air vent above the drain, and also seal the bathroom sink because it connected with the tub drain. This has never failed for me. *I just found an adapter made for this lol...