Hi! This is my first guide so bare with me until I am good at this. BEFORE YOU VOTE OR MAKE COMMENTS ON THIS GUIDE. TRY IT. WARNING: THIS IS AN EXTREMEMLY SQISHY BUILD SO USE HIM LIKE A PUSHER LIKE HE IS MEANT TO OR STAY NEAR TOWERS. CHOOSE YOUR BATTLES WISELY
I chose to make a guide for Viktor because there were no guides that I really liked. I've noticed Viktor's spells really drain his Mana, so I for a way to not only make his Mana capacity huge where he won't run out after taking just one guy out at a time, but also outrageously increasing his bursting abilities! The stats are not completely accurate, having items with passives that increase AP by % will put Viktor's AP up to almost 800!

Pros:

Extremely High Damage (In ability bursts)

Lots of DOTs

2500(give or take) Health

Loads of Mana

Not many people use Viktor(that I have seen)

Cons:

Extremely Squishy

Being good at skill shots is a prerequisite

Get in and out champion

(Slow movement speed.. do lots of damage and leave with flash. If enemy is still alive and chasing, use gravity field and place it over you and keep going. [this also may make it possible to finish enemy off])<--Guess that can also be a pro as well

I usually try to start mid that way I can level up/ farm quicker so I can pull of ganks more successfully earlier in the game. Because Viktor's spells use up so much Mana(more than any other champion I've used), we want to start the game being able to stay in as long as possible before returning back to base to refill Mana/Health and purchasing items. The First Items to buy starting out is the Sapphire Crystal, and Health Potions x2.

So as I have already mentioned in my intro about increasing both Mana and Ability Power, I am going to explain the process here.

After reaching Level 6, you can now combo the enemy.. however, it wont be thaaat strong yet. Harass the enemy enough for him/her to recall. Then you are safe to recall without them having an advantage over you.
1: Purchase Augment Death (1000g +45ap and 30% Death Ray DOT) and Tear of the Goddess (300g after purchasing the sapphire crystal +250 Mana +7 Mana Regen Passive: Every spell casted adds +4 maximum Mana capacity.)
2: Now you can pull your first combo without running out of Mana before you run out. Also adds new Death Ray DOT and gives and extra 45ap. After owning the mid lane, you should be able to gank easily. Once you raise enough gold, Purchase Archangel's Staff (2000g after TOFG. +60ap + 10 Mana Regen +3ap per 3% Max Mana(constantly increasing AP and Mana)

3: By now, you will have amazing burst damages for early/mid game. Beware, you are extremely squishy! My guide is for Summoners who have good skill shots and are able to tell if the enemy champion is gankable/killable. The next purchase is rather expensive, but very necessary to keep up the bursts. Rabadon's Deathcap (3300g +120ap +30% total ap)

4: Now for another Important Purchase.. getting into the 30min mark, most enemies will have raised magic resistance because you keep taking them down one by one(and team fights). You wont be hitting those same high bursts as you used to... right? No.. why? Cause not only your ap keeps raising by each level and every spell you cast(archangel passive), but you're also using none other than the Void Staff! (2295g +70ap +35 Magic Penetration) Each Burst should be hitting 500+! (not to mention, you have 2 DOT attacks as well.. and the slow/stun)

5: I'm getting a little embarrassed to say that I'm getting really excited to finish the build lol. I am just going to tell you that the next and last two items to buy are Rylai's Crystal Scepter (2900g +80ap +500Health and slows 35% movement speed for 1.5 sec.) And Will of the Ancients (+50ap Unique Aura: +30ap and +20% spell vamp.)

For 1v1: Gravity Field, Death Ray, Chaos Storm, Power Transfer, And chase them till they're health drains completely using both DOTs and basic attacks and final Death Ray attack if you're still chasing.
Try to stay way from Magic Resistant tanks(or just proper tanks). With Viktor being as squishy as he is... and slow, more than likely that tank will take you out pretty quickly. Instead of going all out on the tank, harass the live daylights out of him/her and when the tank is half dead, go for it. just make sure you have a good amount of health also.
Also, getting health will be quite easy because Viktor has 20% spell vamp, every hit with all but gravity field will return a huge chunk of health back to him. Also this makes it easier to stay alive during a 1v1.