AP Kog'Maw, mid
Kog'Maw · Mid · AP · by Cowsep Classic
Build
Runes and masteries
x9Mark of Magic PenetrationMark
x9Seal of Mana RegenerationSeal
x9Glyph of Ability PowerGlyph
x3Quintessence of Ability PowerQuint
- Ability Power +25.80
- Magic Penetration +9.00
- Mana Regen +3.60 / 5s
Run magic pen marks with flat AP glyphs and quints because this is the long-range E and Living Artillery poke page that spikes through Sorcerer's Shoes, Archangel's, Mejai's, and Deathcap. Mana regen seals fit the Tear into Shushei's Mana Jug plan so you can keep farming and throwing artillery before Seraph's is online.
Go 21 Offense for AP, cooldown, magic penetration, Archmage scaling, and Executioner so the Deathcap and Mejai's artillery actually punish people. The 9 Utility points are for the biscuit start plus Meditation and Expanded Mind, matching the early Tear, Shushei's, and Archangel's mana-heavy game plan.
Skill order
ultimate at 6, 11 and 16Icathian Surprise
- Caustic Spittle (Q) takes points at levels 4, 14, 15, 17, 18.
- Bio-Arcane Barrage (W) takes points at levels 2, 8, 10, 12, 13.
- Void Ooze (E) takes points at levels 1, 3, 5, 7, 9.
- Living Artillery (R) takes points at levels 6, 11, 16.
Max E first for the unplayable mid lane shove and slow setup, then W for ranged follow-up, while taking Living Artillery at 6, 11, and 16 for the Rylai's artillery chain.
Why these items
Openingsall within the 475 starting gold
Doran's Ring, Health Potion x2, ward, biscuit — Take this mid when you want AP, lane mana, a little health, and enough early sustain to farm until Tear.
Corein buy order
1Tear of the Goddess700gTear comes first because the spell spam drains mana quickly and you want it stacking as early as possible.
2Chalice of Harmony880gChalice gives the mana sustain and magic resist needed to keep laning into magic damage.
3Shushei's Mana Jug2,600gShushei's Mana Jug upgrades the sustain plan with AP, cooldown reduction, magic resist, and more mana regen.
4Sorcerer's Shoes1,100gSorcerer's Shoes add the magic penetration for E and R poke once the mana problem is handled.
5Archangel's Staff2,700gArchangel's Staff turns the stacked Tear into real AP and keeps scaling with your mana pool.
6Rylai's Crystal Scepter2,900gRylai's makes the artillery pattern nasty: once R or E hits, the slow helps line up the next spell.
7Mejai's Soulstealer1,235gMejai's is the snowball buy when you can hide behind the team and collect kills and assists from long range.
8Rabadon's Deathcap3,300gDeathcap is the big AP spike once the poke engine is online.
9Zhonya's Hourglass3,260gZhonya's is the late defensive AP item for when they dive onto you.
Situationalonly when the game asks for it

Rabadon's Deathcap3,300gBuy it for the huge AP spike after Rylai's and Mejai's when you have room to keep poking.

Zhonya's Hourglass3,260gBuy it when divers can actually reach you and you need stasis to survive the commit.
Notes
the author's own wordsThis is AP Kog'Maw for sitting miles back and making the lane unplayable with E and Living Artillery. The early mana is rough once you start using more than W, so get Tear going fast, add Shushei's for the mana sustain and magic resist, then finish Archangel's so every spell keeps feeding the build.
Rylai's is the fun part: hit them once with R, slow them, and keep dropping artillery while they try to walk out. After that, Mejai's is for snowballing from the back line, Deathcap is the big damage spike, and Zhonya's is the answer when Akali, Vayne, Warwick or anybody else finally gets tired of being puked on and dives you.
Source. Transcribed from Cowsep Classic's video AP Kogmaw is his best build in League Classic — https://www.youtube.com/watch?v=z5o-SmiTp3g
The video never opens the rune page, mastery page or skill order. Those follow from the items it does show, and are not the creator's.