Help with Bullhead case back

[doomguy10011]

I was changing my Bullhead crystals yesterday, and the brown one went fine without a hitch. 
My black Bullhead however, has a very tight case back. My local watchmaker couldn't seem to open it either, though he pretty much uses the same tools I do. Hoping to get some advice on how to proceed. Thanks in advance!

[SeikoPsycho]

I use a Seiko case holding tool S-210D. It's placed in a vice and has interchangeable spacers to fit the lugs. It secures the watch case so extra force can be applied to the case back removal tool. I use a three jaw LG case back removal tool. I haven't had a case back yet that I couldn't remove with this set up.