I read some builds about magig pen, and i have tried a full pagic pen heimerdinger.
The first levels are painfull, but teamfights will be ridiculously easy with your sentries and your two ~5sec cooldown offensives spells (with 40% cooldown reduction, that you have to get level 16).
With a full AP build you will be able to deal mass damages, but you will be too squishy to deal that damages.
The best way seems to be building à tanky Heimerdinger while focusing MP, that I'll show to you how I build that.

Only noobishs spells here ;D

Seriously :

Heal : usefull early game, especially if you're solo lane, it will give an awesome surprise to anyone who want to kill you under your sentries. You can also use it to heal your sentries, but only do it if you're not full life. Your ulti can fulfill this role better than Heal, that is better for you and your teammates.

Clarity : another usefull early game spell, especially in case of solo laning, when you have to stay in lane for more gold and exp. If you will be two in lane, ask someone with mana to come with you, like an AD based on mana.


Anothers good spells combos :

Teleport/Heal : they will be far better if you are enough skilled to solo laning and don't waste all you mana.

Teleport/Clarity : if you play smartly and safely, but lack of mana, Clarity will help you more than Heal

Teleport/Flash : Heimy is very slow, and doesn't got any escape mechanism (but got some CC with grenade + ulti). That's why Flash will help you to GTFO from melee to a safe spot where you can throw more spells.

Teleport/Ghost : may works like Flash, but be less effective if catched in melee.


Any combination of these spells will work, but it depends of your way to play. Follow your way, not others like sheeps, but feel free to learn from others !

Maybe exhaust will works, especially for first blood with a lane partner, cause to slowing an ennemy and reduce his resistances by 10.

I got there : 10MP ; 10AP at level 1 ; mana regen per level and AP per level.
I attempt to optimize the spam of spells to decime all your opponents.

Maybe the blue mana regen per level runes can be switch with mana runes at level 1 (~+100 mana), following the same goal.


Why 10MP instead of 14 ? Because of the ennemies MR. Every champion (unruned and unmastered) start with 30MR, and it can't goes negative without Malady or your sentries' shots (that we use latter). So, with your runes and Sorceler's Shoes, you reach the cap of 30MP.
If you often play versus ennemies that got Resistance, feel free to take the 4MP missing, but don't forget that 2MR reduce magic damages by 2% and 6MR reduce magic damages by only 5~6%. (If they got more than 30~36MR at low levels, 4 additionnal MP is better than 10AP, but it will be too late to change ; don't forget, with Doran's Ring and runes, you start with 25AP, that increase your sentries' damages by 5)

It's a classic skill sequence that I follow : obviously, max ulti first as soon it is available, your sentry must be level 5 when you are level 9, you have to unlock all your skills soon (I start with the grenade at level 2 for the blind/stun it gives to you).
All the strategy comes here : you are level 8, and you have to choose with spell you'll max first (maybe both). Grenade helps you to farm minions and CC ennemies, whereas Rockets can easily hit 3 (or 5) tragets ; usefull against fleeing ennemies, but only aim nearest targets.
If you play well with only Rockets or Grenade, you can upgrade only one spell and keep the other for last levels.