The first reply you would have gotten from GMC would have been without a doubt, "We need to see your code." Inccubus, Esco and I can't help you at all if you don't post code.
But even without knowing your code, I can give you some insight on how to program the Skull Knight. First off, his sword is NOT A PART OF HIS SPRITE. Weapons are separate objects; the worst thing you can do is treat a weapon as part of the same sprite/object. For starters, it's a waste of memory, because the sprite needs to be larger to accommodate the extra width added by the sword. Also, some enemies deal more damage with their weapons than with their bodies (or vice-versa, as is the case with Axe Knights and Bone Skels). Second, it's easier to check for collisions with a sword if it's its own object, as opposed to checking the image_index of the Skull Knight. Download the latest release of GMvania (my Castlevania engine) from this forum and look at how the Sword Skel was handled. The Skull Knight attacks about the same way.