Not sure of the thickness of your branch so I will throw out a few ideas. Assume you have electricity:-)
Be careful that the branch does not jump and get ya if you do find a way to cut it up>
Circular saw will cut to about 21/2” deep, if you can get one in there. It don’t have to look pretty, you might be able to go completely around the branch.
A Sawzall, aka, reciprocating saw with a 12” blade might work, just be ready for the shakin.
A hand bow saw will work, if you can find one!
Hope this is helpful
Regards
alfa6 ;>}
Thank you...so much...