If you really need to get serious yet avoid damage, try this.
turn the engine over until one cylinder is at TDC on a compression
stroke, remove injector, pack the chamber with grease, refit
a custom made "injector" to which you can attach an air hose to, connect this up to a air compressor
and wait for the head to be lifted off by the air pressure.
You will have to lock the flywhell in place so the piston remains at TDC.
Depending on how struck it is, this may hours to months but it ALWAYS come apart without damage.
I have seen this work on engines with rusted in pistons, rusted on heads etc.