Greetings, this is my first guide for a champion who I feel is grossly underplayed and under-appreciated. Viktor does it all and he does it with style. Need a quick shield? He has one. Need an effective slow/escape/trap mechanism? He has it. Need a long range poke? He has it. Need a silence that tracks cloaked enemies and does obscene amounts of damage? He has it.
Q - Energy Exchange
W - Gravity Field
E - Death Beam
R - Chaos Storm.
His Q starts off as a moderately good nuke/defence spell that you shouldn't over look. It's not honestly effective enough to level it first but it does good damage and it can mitigate a lot of DoT abilities or damage if used properly. I can't count the number of times it's saved my life from being ignited by simply hitting a minion with it and backing away as such DoT effects always burn away at your shield so when in doubt, use it and pop a potion.
His W, a wonderfully effective anti-ganking took and all round effective safety mechanism is one of the best things about Viktor. It can cripple tightly packed champions or make it safer to get in, nuke and then run to the safety of your turret.
His E is his bread and butter. No other ability gives me the same giddy feeling of glee as watching it strike enemy champions, particularly when you score multiple hits and if you have the Death Augment, getting kills from the DoT effect. A fully built Viktor should be outputting 700+ damage per strike and when you factor in the DoT and how quickly it recharges, he's able to put out a staggering amount of damage over a sustained time. With the slow from Rylei's, it can help slow fleeing enemies enough to coup-de-grace them with your Q or R if they've too much health for your Q. Or both if you're a fan of overkill.
His R is a simple AoE nuke that can silence for a second and a half and does an absurd amount of DoT, particularly in conjunction with his other abilities like his W which can prevent them from running. This ability to abruptly change the tide of a team fight or bring it back to the tipping point simply by the sudden amount of damage and continuous damage. Don't put it down prematurely though. It's cool down is quite short at later levels but it's best, when you're in your earlier levels to use it sparingly or if you just want to scare off the enemy mid. Often this spell plus his E is enough to scare off most other champions who will simply not want to risk giving you a kill and keep in mind, Vik's E is a fantastic finisher for the unwary. I've killed enemy champions plenty of times who thought they were safe behind their tower and underestimated his E's range.
Viktor also has a unique item, his Hex Tech Augment. This item can be upgraded with three different augments. Augment Power buffs his Q, gives you a little more health, health-regen and upon using your Q, you get a short speed buff. Worthless. His second is Augment Gravity which increases the range and effectiveness of his W. Should you ever be supporting someone with Viktor, this augment gives you cool down, increased mana-regen and mana storage and is moderately useful. Honestly though, I'd still not recommend this even then. The only Augment you really should bother with is your Augment Death as your E is your core skill and it's what you should be focussing on. This augments gives you a staggering 30% additional damage which, if you're doing 700+ damage means you'll also be inflicting 210+ damage across four seconds which, when you can do this a second time in about six or seven seconds, that's nothing to be scoffed at.
Build order - Play guide.
How I usually start is with Meki's Pendant for mana regen because Vik is very mana intensive and a pair of health potions. Simply put, play passively till you have a point in your E, W and Q in that order. If you're playing against someone who doesn't have gap closers or quick teleports, go E > Q > W but it's safer to go E > W > Q in case of ganks. It will save your life. When you have all three abilities it's time to grow a little more aggressive. At every opportunity, throw your Q at the enemy champion and at least one or two basic attacks. At the first few levels, basic attacks, even for squishy mages can be the difference between a kill and an escaped champion. Also try to angle your E at your enemies as well. With the shield shaving off some 40+30% your AP bonus of damage, that can make all the difference in an early damage exchange and the second that happens, make sure you pop a potion into the mix. Two minions and the cost of the potion is returned while the enemy champion has to return to their turret to heal up or back to their base.
After you get 850 gold, return to base to buy Chalice of Harmony and basic boots. If you have a little extra, have at least two potions but I try to keep four on me at all times. Return to your lane and continue farming and harassing the enemy champion where appropriate but always be careful of a gank as at this point, if the enemy has a jungler, he should be making himself known. The last thing you want is to be caught flat footed as the enemy jungler pops in to say hi.
If the enemy champion is chasing you, it can be handy to drop your W at the edge of your range so he or she walks into it and letting you get a few hits before backing off. If you can't outright kill an enemy champion, don't bother chasing. Wearing down your enemy and then farming when he's backing is best. You don't want to be too greedy and hand the enemy champion unnecessary kills.
When you have a thousand, get the Augment: Death. Don't bother with the other two. Compared to Augment: Death, they're useless. Then get Rylei's Sceptre. If they enemy champion is doing a fair bit of damage to you, get the Giant's Belt first, if you're doing a lot more damage and generally coming out on top of any fight with the enemy mid, go for the Blasting Wand first. When you finish the scepter, finish Athena's Unholy Chalice and then go get the Deathcap. This should help maintain your mana, allowing you to spam out your spells faster and you're going to need the rapid mana-regen with the shortening cool down and the Death Cap will take your threat level to the enemy team something fierce.
Now, your last item is entirely down to how you're doing. If you're finding that you're coming away from fights heavily injured or you need more heath sustain, get Hextech Revolver and upgrade it to Will of the Ancients as soon as you can. This should bring your AP level to the upper 700s and give you fantastic health-regen thanks to Viktor's massive damage output. If the enemy is too tanky and you're not doing enough damage, try either a Void Staff or Abyssal Sceptre. The Staff grants you a lot of MR mitigation but it doesn't offer you much in the way of defence and keep in mind, this means you'll have to return to the base more often to heal up. The Sceptre will give you a bit more defence against magic damage from enemy champions but the MR penetration is not quite as good.
I personally prefer Will of the Ancients for the simple reason that the health-regen is just inherently handier and should keep you dandy if you get hit with an attack or two. Be sure to stay out of the combat as much as possible. You're a sustainable-high-damage mage and you should be focussing on nuking the enemy AP and/or AD carries and then the tanks when the serious threats are dealt with.
This is my first guide so please be gentle and yeah, I know this lacks a lot of the polish of other guides this should still be a handy guide for others who wish to try their hand with Viktor.