This is the chaingunner Kog'maw.
Why chaingunner?
Cuz you attack very, very fast and you do a lot of damage.

It's about 1400 damage per second with full equipment at lvl 18.*

* Enemy with 3000 life points and 30 MR.

I think that is the best summoner spell, slow's, decreases the enemy's damage and don't forget that is also decreases the enemy's MR. It's excellent for killing and good for escaping.

Everybody is using it, so we too. Excellent for escaping and maybe to get kill's.

Sometimes I take this summoner spell instead of exhaust, cuz you can run away or follow you enemy much faster, don't forget to change you masteries when you take ghost!

Caustic spittle: Increases your attack speed and decreases the enemy's magical resistance and don't forget it does damage. Before you start shooting at your enemy with Bio-arcane barrage use this.

Bio-arcane barrage: It gives you more range and massive damage if your attack speed is high enough, cuz you turn 6% of your enemy's max. life into magical damage. And the question is, i think you know it, why can we easily strike down tanks?

Void ooze: That's your slow, it also does a little damage like Caustic spittle and it's a deadly weapon combined with your other spells. Try to slow your enemy's, then hit them hard with your Living Artillery or Bio-arcane barrage.

Living Artillery: Not your best spell I think, but nice to harass and to kill the low lifes who try to run away or who just out of range.

This is one of the most important things.
First i have to say that there is a big difference between magical penetraion and reduction.
With magical penetration you just can reduce his resistance to 0.
With magical reduction you can reduce it below 0.

For example:
The enemy has 30 MR, 23% less magical damage.
You can reduce it with your to 25.5
Your runes give you additional 14.22 penetration, so his resistance is 11.28
When you buy and hit him 4 times, his MR is below 0, at -12,72
Finaly you hit him with and his MR is decreased to -37.72!
So he takes more damage, how much?

2 - 100 / ( 100 -(-37.72)) = 1.274

He takes 27.4% more magical damage instead of 23% less and that is one of the reasons why you do so much damage.

Let us talk about the damage.
Kog'maw hardly does normal damage, it's about 100/attack but with your Items and you deal additional magical damage with every attack.
For example:

A random enemy hero with 30 MR and 3000 life, i think that is realistic for Ashe in lategame with an defensive item that gaves her a few life points.
Her MR is -38 against you, so you do 27% more magical damage.

Normal damage: about 90 with an activated per attack
Magical damage (per attack):
20 MD
120 MD (4% of 3000)
25 MD (100 with every 4th attack, 100/4 = 25)
180 MD (6% of 3000)

So you deal 345 MD/attack plus his negative MR, +27%
Including the negative MR of your enemy you deal 438 MD/attack + normal damage and that's quite a lot when you know that you attack 2.5 times/second.

All in one you do about 530 damage/attack.
So you need about 2,2/2,3 seconds to kill somebody with 3000 life and 30 MR.

That is your first item, it helps by killing creeps, gives your some life and lifesteal.
Boots, i mostly use this one cuz you can hit your enemy's better with and . When you are mid against Annie, Brand or something else you can also buy .
Malady is the perfect item for Kog'maw, Attackspeed, MR reduction, MD/attack and some AP.
Very expensive, but like must have. The increased attack speed and 4% MD of enemy's life with every attack is excellent for Kog'maw.
Maybe not the best but very cheap, 60% increased attack speed and MD/attack. The active effect is nice too and I use it when i can but armor penetration is nearly useless, only the anti-dodge effect is useful against some heros like Jax or Sivir.
I'm still sceptic cuz of the Critical Strike chance, but the increases movementspeed is very nice and attack speed is excellent.
A very important item for Kog'maw, more Mana for artillery and you don't get CC'd so easy, alternative you can buy for more life.

Pros:

Excellent damage
High range
One of the best mid/lategame carry's
Can go mid
Tank buster
funny dance animation! x)
funny skin's, rocket artillery! x)

Cons:

Squishy
Low basic movement speed
weak in earlygame
victim against invisible heros like eve, twitch and some more...
You are always hungry! x)