Go get Intelligence in the Cunning tree right before Thunderlords Decree or Stormraiders Surge or Windspeakers Blessing. Don't take Precision as you can get way more Magic Pen from 9 Marks of Magic Penetration.
You can get The 9 Greater Marks of Precision but you'll get less Magic Penetration from the Normal Marks of Magic Penetration.
So basically the Mastery Page IS (I chose stormraiders surge in this case. All you really need to do is just change stormraiders surge to thunderlords decree.)
Wanderer or Savagery, Prefer Wanderer since mid laners roam a lot when they have the chance.
Then Assassin over Secret Stash. If you have good laning or even trades take Secret Stash. If you get out-traded a lot take Assassin since it's a 1v1 lane so you can stay evenly with their damage.
Get 5 points in Meditation or get 4 points in Meditation and 1 point in Merciless. I take 5 points in meditation since some mages spam abilities a lot and need good mana regeneration to back that up. Yeah you can play a champion that get's their mana back after killing a target with the certain ability blah-blah.
The reason why i don't get Merciless is because most of the time in Late Game or Mid Game most enemies are at low health before you can kill them because of trading or some fighting.
Damn sometimes even your full combo will get them below 40% health or even kill them before you can chase them down or they fight but you have the increased damage because of merciless.
Dangerous Game or Bandit is a tough one. You can choose bandit if it's a really "i'll keep punishing you for farming" Lane. You still get a decent amount of gold but Dangerous Game Saves lives. I don't know how many but ignite or damage over times or even a god damn minion would kill you if you had bandit and at really really low health.
Dangerous game restores 5% of your health and mana when you kill a champion or get an assist. The 5% health gain can save you from the stuff that can kill you after the enemy dies. Mana on the other hand can save you from the enemy chasing you or a sudden 2v1.
Now Precision or Intelligence. Get ready for some Math!
precision is 6 Lethality at level 1 and stays as 6 lethality over the game unless if you buy lethality items, and an extra 1.75 Magic Penetration. After that the next 17 level's you'll gain 0.25 per level Magic Penetration.
If you have the 9 Marks of Magic Penetration you'll get this
0.87 Magic Penetration in fact. 0.87 X 9 = 7.83 Magic Penetration.
7.83 + 1.75 = 9.58. Sometimes they'll round it so it says 10 but it's that number anyways.
Apparently when i go into game. It only add's 0.25 Magic Penetration and not that 1.25.
With the rounded 10 Magic penetration from Mark of Magic Penetration Assuming you have them in your rune page.
( 0.25 X 17 = 4.25 ) + 10 = 14.25 Magic Penetration.
Without the 1.75 and Precision Mastery you're going to have 7.83 magic penetration or rounded to 8 magic penetration.
14.25 Magic penetration isn't actually as much as the 15% Sorcerer's Shoes Magic Penetration. This Magic Penetration and Lethality are FLAT Penetration and not percentage. Assuming that you're level 1, 10 Magic Penetration is actually pretty good since most mages at level 1 are around the 30-40's.
If they have 6 Scaling CDR glyphs and 3 flat Magic resist they are going to be around 34-36 Magic resist. sometimes lower than that. if they have 9 flat magic resist glyphs they will be over the 40's. Since you have 14.25 Flat magic penetration your abilities will do damage like they have magic resist around the 30's - 36's
In my opinion i'll take intelligence with any type of mage anytime. I only take precision for the Ad's or by accident.
Then get whichever keystone you need Thunderlords Decree or Stormraiders Surge or WindSpeakers Blessing.
"ok but don't you want the penetration so you can activate Stormraiders Surge Faster or have Thunderlords Decree do more Damage?"
In my response yes and no. It is nice to activate it faster and do more damage. But at the cost you're also doing less Damage Per Second. Using abilities more assuming you don't miss them is much more efficient then doing more damage for things that you can miss.
Even though the more you can use the more you have the chance to miss. Your other abilities can help that since THEIR Cooldowns are also lowered.
So either more damage. Or more Damage per second. I'd take Damage per second.
I actually had to go into a practice tool game and had to do the math by Guess and Check.
Well thanks for those people who read this all and maybe even upvote or just completely disagree, either way thanks or "okay" and i hope you have a nice day!
), and hits the max 40% CDR very early. Otherwise, the extra MPen is much more useful as a bursty mid laner (like 
, or 
, or 
)
), while those who can one shot at ease should take the extra pen (
)