If you have a vice or workmate, put old inner tube around the air can (fold it in half first if you need a bit more thickness) then put the ends of the tube (not the can) through the vice jaws. A little downward tension so it’s all snug as you tighten it, though not enough to draw the shock into the jaws, and then use a large screwdriver / metal bar through the eye to unscrew it.
Course if it’s only “loosely” on you could probably leave it on the bike and do the same thing with inner tube and molgrips. But the jaws never go on the can.