Here's an idea. If you go with a 10-0-20 mastery setup you can get the 15% spell pen AND the bonus AP while ignite is on cooldown. The only reason why I'm proposing this idea is because the 21st point in utility reduces the time on your summoner spells by 15%, this means you have the extra AP for 15% less time. Just an idea, check out my build too: http://www.mobafire.com/league-of-legends/build/leblanc-the-mage-assassin-29170

Usually the extra AP is not needed on a level 3-4 gank. So after ignite is used, you can do 30% more damage on all spells at level 4 with the extra AP (I checked this in practice). This I think is better than knocking off 4.5 magic resist from the enemy (15% of 30 = 4.5). However, the 15% may do a lot better for you late game vs an enemy that stacks MR. So to me, either choice is a toss up.

Except most, if not all champions have a built in 30 magic resist right off the bat, or about 23% damage reduction. Early game, the mastery would reduce it to about 20%. While the difference may not seem like that much, consider that you normally don't use Ignite until after you've harassed them a few times, making the bonus AP somewhat moot since you won't have much use for it until after Ignite kills them.

Reason I use the 10 ability power is because it helps your early game which i believe is crucial. Magic pen. is only active when the champ you are fighting has an item equipped with magic resist which is rare in early game. So ability power will be more useful

dude ur funny and good build and mmmmm really good tips so thx i think it will help me

Any particular reason you're getting the +10 ability power on Ignite cooldown instead of the +15% magic penetration in your masteries?

Other than that, it's a good guide. In-depth gameplay explanations, decent item build, good skill order. Gotta try out Teleport sometime as well.