In the last few combat games I've played, I've noticed that players with good aim succumb easily to a skulk that can either bunnyhop and ambush well or do a weird curve in mid air while leaping. The latter seems to be effective in dealing with marines that are camping long hallways or rooms with high ceilings.
So try this. Get leap as your first upgrade for skulk. Look for a marine that is just camping somewhere and parasite him (or wait until you hear him start running but be sure to hide somewhere). Now when you think he's coming around the corner (or if he's just meditating
), leap and try to move your mouse and strafe key (sort of make a horizontal parabola in midair) in the direction he's coming from and try to bite him. He'll be so suprised at how fast it came that he won't have any time to react. This move is kind of hard to describe (you gotta see it to know what i'm talking about) but it's been **** me and a lot of other players off.