Pand, I finally watched the video....
You simply aimed and hit in the wrong spot, the fuselage is the toughest part of the aircraft. The Betty will go up like a gunpowder with a few 50's to the wings or wing root. I killed six Betty's in a little over two minutes using only 300 rounds of 50 and 30 rounds of 20 mil.
Try aiming for a wing or wing root and you will have much more success!
The Betty is by NO means invincible....
Strip