Hose it with PlusGas and then start undoing it.
What will normally happen is that it will go tight after a while - this is why people shear things off on old Golfs.
There's probably corrosion on the threads that have been exposed. See if you can get the PlusGas in to the top of the bolt to get in the rust, then work it so you go undo a full turn, back up half a turn, undo a full turn, back up half a turn. Re-apply the penetrating fluid while you're working the bolt out. This should help clear the threads and keep things moving.