It has been a while since I've written a guide, so bare with me here! But I think Viktor is in need of one! I've been reading the forums and seeing a hell of a lot of people saying Viktor is the most underpowered champion to date! They are wrong IMO. Viktor is one beastly mid AP hero, and Im here to show you how i've achieved consecutive Viktories! (Heh see what I did there?)

For those on the run. Needing a Quick guide, here are some points to make your Viktor games work for you.

Doran Ring.
Max E by level 9.
Grab Augment:Death.
Magic Pen Boots.
1 Shot line of minions.
Poke Enemy with E til low.
Flash in - Drop Ult - Q - Ignite = Win.
Team fights hold back til fight starts. Drop ult. = Double/Penta Kill.

Seeing as a lot of people don't think Viktor is a viable AP caster choice compared to others, heres a list of pro's and cons that are unique to him.
PROS

The Safest Farmer in the game.
Very fast farming by level 9.
Amazing Nuke Poke.
Great Pusher.
Awesome CC.
Sweet Unique Items for cheap AP.
Good Escapability.

CONS

Long CD's.
Hard to Land your poke sometimes.
Shield Delay.
If you have a bad game, you will be useless.

RUNES
For my runebook I take 9 Greater Mark of Magic Penetration pretty standard for an AP caster. A bit of Magic Pen won't let you down against those tankier opponents.
I take 9 Greater Seal of Scaling Mana Regeneration this is because I believe Viktor is fairly mana hungry, he's not overly hungry but later game when you can 1 Shot minion waves you will be hitting you E quite often, to Poke the enemy and clear minions. This helps sustain your mana pool.
Then 9 Greater Glyph of Ability Power, these are just for a bit of bonus AP to start with, giving you a nice advantage of power over people with less AP.
Finally 3 Greater Quintessence of Ability Power, for the same reason above. Just that extra bit of damage early game helps so much.

MASTERIES
For masteries I go 21/0/9 this is because I tend to build Viktor as a burst/nuke AP caster, I have tried 9/21/0 and believe this is also viable, especially in lanes. But my preference is the more AP for mid lane. The main reason for the 21/0/9 is because I feel with flash and your large AOE stun, you can almost always escape death without the need for quite tanky masteries. Also taking a point in the Executioner masteries with benefit your lazer and Augment:death greatly. Basically giving you 36% extra burn on your lazer when enemies are low!

My item build is shown at the top, minus AUGMENT:DEATH because I couldn't find it in the item list! Here is the explanation.

Early Game

Doran's Ring

Starting with this gives you a much needed AP boost at the start, and that all important mana per 5.

Moving onto the next CRUCIAL item is the
AUGMENT:DEATH.
Like I said no image for some reason! Regardless, this is amazing later on, gives you a nice hefty AP boost as well as a nice dmg bonus to your E skill. Makes farming so much easier and safer.

For boots Viable choices:

Sorcerer's Shoes
My personal choice, as most games I have no need for any other boots.

Mercury's Treads
Giving a nice bit of MR and some decent CC escape. Good if you are getting dominated min lane by a caster.

Boots of Swiftness
Only viable for catching those faster opponents with you long poke. But I favour the Magic Pen over the extra 1 Movement speed.

Mid Game.
After getting my boots, augment death and dorans I start to build towards my main items. First off is the Rabadon's.

Rabadon's Deathcap
A great item for Viktor. A lot of AP, and giving you a 30% bonus to later items. (Including Augment Death!) A must have in his build IMO!

Rylai's Crystal Scepter
This is also an amazing item for Vik. Gives him a nice HP boost for survivability. But that all important Slow and his spells. In team fights drop your ULT and this will CC the hell out of the entire enemy team!

Zhonya's Hourglass
A nice Armour boost to counter those pesky carries. Nice AP boost, but mainly for the amazing active it brings. Helpful in so many ways. I added this to my build since I laned against a Karthus, every team fight I would clean up the enemy team, be very low on HP and he'd Ult. Pop'd this and it saved me so many times. Obviously helps in so many situations! Learn how to use it!

Lich Bane
Movement Speed is a bonus, MR also a bonus. But this item is amazing for harass/Nuking. Cast any spell and will deal a X2 of your base AP on your next auto attack! Make sure you save it for enemies, not minions!

Other Viables

Morello's Evil Tome
Good AP, amazing CD bonus and some Mana per 5. Can't go wrong really! The CD reduction really helps Viktor as its one of his weak points!

Void Staff
For those tankier opponents. Self explanatory really! Swap it out for Lich bane if their team is MR heavy.

Will of the Ancients
I like this, and actually swap out Lich bane for it sometimes! The Spell vamp is really nice on Viktor! The aura is pretty sweet if you have other AP casters on your team.

Abyssal Mask
A really nice item for Viktor. Would take it if I had another slot! Very nice MR and AP but the passive is great for late game team fights. Helps your ULT shred through enemies. Buy if the enemy team has a fair bit of AP.

Banshee's Veil
Purely situational, if you are getting mobbed by AP casters in team fights this is pretty nifty. I prefer Zhonya's over this though as that allows you to use it when you feel the need.

Rod of Ages
An item I personally hate for a lot of champions. Its over-priced in my opinion, although it gives some nice survivability, AP, and mana pool. If you are looking for a bit more HP/Mana early one grab this before anything else. You can always sell it later!

Deathfire Grasp
A pretty decent item, but overlooked by the usefulness of the others! If there are HP heavy opponents, feel free to take this to burn their HP. Using the active is a nice added damage for your burst. Very Veigar-Esq!

Viktor is pretty simple to use in my opinion. All you have to do is harass the enemy down with your long poke (E Skill). Til' they are low, flash in and burst the rest of their HP down!

Early Game
Early game you want to play fairly cautious. Keeping out of range from enemy spells and try not too overextend so you don't get ganked. Ward both bushes if they have a jungle, and watch the enemy if the favour towards one side, probably means a gank is incoming.

Keep on poking the opponent with your lazer to keep their HP low. Make sure you don't run out of mana though! Pace yourself and you will be fine. When you have around 1.5K. Open up on all your spells and try to bring the enemy low, or even kill them. So you have time to head back and buy your first items without the enemy team pushing your lane.

Mid Game
Once you get some items you should be hurting the enemy with you E skill. Try not to spam it, but wait till you know you can land it for a bit of harass! By level 9 you should have your E maxed out! And with Augment death you will 1 Shot minion lines! (Apart from the big ones)

Keep farming safely and if you have an opportunity to kill. Take it.

Late Game
Keep up what you've been doing so far. Landing as much poke as you can! Wait for a team fight to erupt and hold back! Wait til alot of the enemy CC has gone off and dive right in! Drop your ultimate as soon as possible and try land your AOE stun on as many as possible! Target the enemy carry preferably, drop him fast with all your spells. Let your ULT clear up the enemy team and finish them off with your long range poke. Remember to land ignite on any healer in the fight!

I will be added new screens later on.
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Top game my team pushed and won in 16 minutes. Hence not many kills!
Second game an Ashe and Panth fed the opposing team like mad. Me and an Udyr bought down the entire enemy team and won eventually.

Here are my stats from that game!

I know its not a brilliant guide but if you think there is anything I can add please feel free to comment! :D

Any feedback will be amazing thank you!

Have fun playing VIKTOR!