What I did was spray some WD40 at the top of the piston the night before. Then the next morning I used a 24" pipe wrench, and it came off within a minute. Bear in mind that this chair was 17 years old. So just get the biggest pipe wrench you can.

I also replaced the cylinder with this one from Amazon ($15). It seems to be a good match for the OEM one, but costs a forth the price. https://www.amazon.com/dp/B00JQQYNU2?ref=ppx_pop_mob_ap_share