I've no idea whether it's the quickest way to do it, but last year on the October trip we got a train from the airport to the main Cologne station, then took the U-Bahn out to Bruhl to get the Phantasialand shuttle bus (well, technically we didn't go straight from Cologne to Bruhl as we were staying at an Ibis in Cologne, but it was near a stop on the U-Bahn route you'd take anyway).