The better your magic stat, the more chance you have of drawing Demi successfully from Diablos. I usually fight him as soon as I get the lamp but one time I forgot and didn't do it until Deling City. At that point, my characters were exactly how I wanted them to be and I drew about 3 Demis every draw attempt. I haven't tried Draw--Cast Demi on Diablos, I've always instinctively gone for Stock but I guess it would work both ways.