Kog'Maw is a hugely underplayed adc, and with the appearance of Braum, not many adc's are really that strong. Kog'maw is a perfect Braum counter, he comes with damage that does a % of max health, so tanky Braum can say goodbye. His ult is very hard to block with the Braum shield. Kog'Maw brings strength to a teamfight with a high amount of attack speed and range, providing his W is active (Costs No Mana). Kog'Maw is a very high skill level adc because his positioning in a fight is key to his kit. He is very farm heavy and does rely on items to do his damage.

I usually take Flash + Teleport on Kog'Maw. You may say "Why take teleport ?" Well, teleport is a good summoner spell with Kog'Maw, as he is very farm dependent and quite squishy early on, so teleport allows you to go back, get an early cutlass and then keep farming till you hit Blade of the Ruined King. When you pick up the BORK you will be able to duel most adc's solo.

To go into more depth with the items, Blade of the Ruined King is a MUST buy it gives Kog'Maw the ability to confidently duel most adc's straight up, a exception here is Vayne. The Trinity Force will benefit Kog'Maw almost as much as Blade of the Ruined King, it gives him ap and mana from the sheen, extra attack speed and crit chance from the zeal and the phage gives you sustainability along with greater kiting ability from it's passive. Infinity edge helps Kog'Maw to do additional damage, as he is quite weak early, you build what I call Crit'Maw (That play on words, kek) When Kog'Maw can crit 85% of the time and do 6% of max health, he can shred tanks instantly. The Statikk Shiv/Phantom Dancer will also give Kog'Maw crit chance and more attack speed. With a full build Kog'Maw, you can shred a 4000 health Vladimir in around 4-5 auto attacks.

Kog'Maw is very good in lots of ways, he brings a large amount of damage to easily take down tanks, is a great lane and tower pusher, and your ult looks awesome as hell if you land a snipe and get a kill (kek).
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His disadvantages are quite large, he needs a poke heavy support who also has a good initiate (I recommend Sona or Karma) He needs perfect positioning and is of a high skill level.