Co2 is fine with a Shocker, but it invites a few problems. You must wear gloves while shooting it or risk having fingers stick to it due to icing. Icing is nasty when it happens. Also, co2 leaves more residue on the internals which you must clean off frequently to insure optimum functioning. Also, if you're using the standard manifold Maxflow, some of them have a nasty habit of cutting orings; switch to vert Maxflow if at all possible.
Or avoid all this stuff by using air.