Since I got Dragonball Xenoverse on steam, my index fingers have been pretty sore from having to hold the triggers most of the time. [spoiler]I've got a used chair that cost $5, it's honestly the comfiest chair I've ever sat in.[/spoiler]
Upper back or shoulder.
I play on my bed, lying on my belly, and hold up my upper back with my elbows and forearms flat against the bed.
I've tried playing sitting down (on an office chair) and I can't do it -- my skill is suddenly cut in half for some reason.