What's the trick to removing the cover plate at the back of the diff?

I removed rods attached to the center. Drained the old diff oil, it was very black. I removed all bolts attached to the cover plate, bastard still would not move, I then used a flat blade screwdriver to lift the cover off, still failed to dislodge. I gave up and redone the bolts and refitted rods.

Pumped about 1.6 liters of diff oil , went for a test drive, diff a bit quieter. Discovered oil dripping between diff and diff plate, I must have warped the plate whilst using the hammer and flat bladed screwdriver.

Hence the question, What's the trick to removing the diff cover plate without warping the plate?