Well people will say that the closer the creature is to your level the more skill point experience you will get per kill. This is true, however, this is not the best way to get skill points. To do that, you need to find the creature that will give you the highest ratio of skill points experience to normal experience. This will take some patience, but usually a green creature will do the job. Let me give an example. When my sorcerer was training from level 31 to level 32, I found that sand golems (which is green) gave the highest ratio of about 3.75 skill points for every percent experience.
Hope I Helped