Penetrating oil - the acetone/atf mix is supposed to be the best - and let it sit for several days. Heat the cylinder with a propane torch. Heating/cooling cycles with penetrating oil making its way in there is what will do it.
Do not try to use force! Only a light rocking back and forth. Force will break the piston. Remove that wrench and fasten down the cylinder again so it won't move (It bothers me seeing tools abused like that).
I've freed up stuck cylinders and found they were still serviceable. Putting a lot of force on the piston could very well break a ring land or bend the rod. Don't go there!
It won't move till the rust has been penetrated and somewhat dissolved. So apply a large dose of patience and realize this won't come free in the next hour - more like next week.
Wanted: Dead, smashed, crashed or trashed gauges
BMW GAUGE REPAIRS - TACH*SPEEDO*CLOCK*VOLT METER *PODs & LIGHT BOARD*