This is my AP Kog Maw guide which should help point out some things about how AP Kog sould work and reccomend a build i currently use.

Pros Extreamelly high damage if played correctly Awesome range Good harrassment Easily able to bait people into a false sense of safety Will pretty much always do damage to anyone(tanks included)

Cons Squishy No real escapeability Rellies on skillshots in many cases

General

• Ability Power: It is my first pick for Quintessences and Glyps as they will give you the best possible AP and since having a good starting AP can help you at any time.
• Magic Penetration: Picking Marks with MP will help you gamage tanks and destroy squishies early-mid game
• Mana Regen: Using Mana regen runes will help u harrass your ememies more while haveing to go back or sit idle for less time so you should choose some Mana Regen Seals

Pick Doran's Ring and head for your lane(Mid reccomended).Keep in mind that Kog Maw can DPS properly after level 4 as you will need all of your abilities.After you have farmed enough Go back and buy Boots and Blasting Wand to boost your damage.If you have farmed 1600 or more gold pick Needlessly Large Rod and go for Rabadon's Deathcap instead of Lich Bane.After returning to your lane and farming some more Return and buy Sorcerrer's Boots and whatever else you can Depending on what you have previously bought.Now its time to start ganking other lanes which should usually be done when the person in mid has left or is missing.Make sure you dont waste too much time ganking as you will need the exp.After getting Archangel's Staff you should consider spending some time in your base to charge it up using your ult and utilising the well's mana regen BUT this should only be when the game is going well.After you get Rylai's Crystal Scepter your ult is a constant pain for the enemy team especially considering its insane reach.Finish up with Archangel's Staff or Will of the Ancients for better survivability or damage depending on what you need.

Ignite:This can be replaced with something of your liking even though Ignite is higly reccomended
Flash:This should only be replaced with Ghost so as to give kog some extra escapeability.

[1]Kog Maw can get very deceptive especially when he has

Lich Bane and can benefit from it.
[2]Your priority list should beand so on until you can repeat again.
[3]Blue Buff is Your best friend.
[4]You should consider your ulti's increasing mana cost a threat and should only use it once or twice until the debuff fades.<--This doesn't apply to blue buff holder's