Spray the cap seam with WD40 and wait a while. Now heat the reservoir end with a heat gun or torch. Once hot hold the shock in your hand and strike around the side of the reservoir end with a plastic mallet. The cap will make its way out far emough to grab it with a set of grips and remove.
Alternatively follow all the preliminary steps but drill a small hole in the cap and pressurise with a rubber tipped air gun.