Viktor is a high damage mage, with enough survivability if is played correctly, and tons of damage on every enemy in teamfights. Just some exercise is enough for you to pwn everyone!! I 'm gonna write only some things about him but i think it's helpfull stuff. Hope my quide will help you :)

pros:
really high dmg
very good early and late game
survivability
supporting your team
good CC
tons of damage in teamfights

cons:
hard to success your abilities
hard to success combos
requires skill to take an early kill
low dmg/difficult to last hit/farm

Viktor is really good in ranked games, because as you can see, he deal really high damage, he got 3 AoE skills so he is the terror in any teamfight, following my build will give you sirvivability so you will not be that squishy, it's hard for the enemies to gang you, and if they do you may take a kill OR EVEN TWO (!), than die. If played correctly, Viktor should scare every opponentto approach him :D

You have to get fed from the minions!! It's the most important thing for every carry. Being in the mid lane there is not competition for the last hitting and you have to take advantage of it. After get fed from the first creeps,
Death Ray will be enough to kill a full army of minions!! :D If the jungler allow you, you can sometimes kill the wraiths, and the wolves. Of course blue buff should be yours every 5 min (If only the jungler isn't Fiddlesticks and other champions like him, who are useless without it).

Well, maxin' your Death Ray 1st, is a nightmare for the enemy. Spam it and harass him and you'll always be higher lvl ;) He's totally dissabled *no farm, no xp, no kills, NOTHING!! Before level 6, after you've pwned him with Death Ray, you can Flash on him and fast Ignite and Power Transfer. You mustn't let the opponent forecast your moves, you have to be as fast as Master Yi with 6 Phantom Dancers and ult improved to lvl3!! ;d
Same SH*T after 6. Spam Death Ray, Flash and there.. you'd better use Chaos Storm 1st to silence him so he can't Flash, Heal etc, then fast cast Ignite and Power Transfer and... HE'S DISAPPEARED! ;D

Gravity Field, is most used when you're ganged (early levels). Don't Flash away when you see the enemy coming. Move smart among and behind your minions (that's about the ganger if Lee Sin gang you, you have to go behind a minion you know..) and if he still catches you, place a Gravity Field and face him (Important: Use Power Transfer!) till he gets stunned. Then you are free to use a Death Ray too and if you see him low, go Chaos Storm and maybe Ignite too (you have to estimate the damage). Over that, Gravity Field is usefull on teamfights (place it in a good position) or you can also slow/stun someone chasing you and save a teammate's life :)

Build's 4 1st items, including your passive item (improved), are enough to pwn every enemy in an early fight. Each of them is really important and you shouldn't skip even 1 Doran's Ring to success. Later, Rod of Ages, give you enough dmg and HP to go on for the next Levels. Rabadon's Deathcap give you the max dmg you could deal and you're gonna make mad every oponent you face (Here you should sell the 1st Doran's Ring)!! You can then choose between Rylai's Crystal Scepter (should be your 1st choise), or if you get hard countered by a mage enemy, you can go for an Abyssal Mask (Time to sell the 2nd Doran's Ring). Now... if you PWN everyone easily, but 1 of them (or maybe more) got huge amounts of magic resist, grab a Void Staff. REMEMBER: Void staff is bought only in case you are not "easy target" for an enemy. Focus on the 1st 2 items i sugested. As a final item you should choose between Abyssal Mask (only if you bought Rylai's Crystal Scepter as 5th item), or Void Staff (only if you bought Rylai's Crystal Scepter as 5th item AS WELL), or you can buy Will of the Ancients if you're going well. Not really suggested, but if the enemy team contains a heavy physical dmg champion, you can pick up a Zhonya's Hourglass :)

I think that maxing your Death Ray gives you the most damage output and you also harass the enemy and farm your lane with the max attribution. Your Power Transfer got enough dmg it's own and should be used to finish off the opponent (if not for the shield),so Death Ray is your main skill. Gravity Field is not really important and every skill point you give it, it still the same, just the cooldown changes but you won't have serious probs with it ;)
And finaly Chaos Storm... not enough to say. AN ULTIMATE REMAINS AN ULTIMATE :D (particularly when it's op like that!!)

I wouldn't suggest you something else than Flash and Ignite...
They both help you kill, i mean, as you saw before, [[flash is included to my combo.
Ignite offer you MORE (YEAH MORE) DpS (Damage per Second) to your combo.

Think:

Chaos Storm+
Ignite+
Improved Death Ray (offer dps)

I take early game runes, as i believe early game is the most important part of the game. Starting good ensures a good late game from you. Combined with the Masteries you will not run out of mana and you will deal tons of damage even pre-6. You can change your Greater Seal Of Replenishment and/or your Greater Glyph of Ability Power with per Greater Seal of Scaling Mana Regeneration and Greater Glyph of Scaling Ability Power. I prefere flat stats.

These masteries are the most "common" for a burst damage mage. Focusing on AP and m.Pen, and combined with your runes your damage output will be IMBA! I 'm not completely sure about my Utility, I just know i need that
Expanded Mind / Meditation so i don't have ANY mana probs!!

First of all i hope my guide will help you.
I'm waiting feedback from you!! Like, upvote and maybe share if you find that staff usefull.
If you have any Questions, please leave a comment below. I will answer all of you :)
Peace and Love from WiZ4RDx and remember...
DESTROY, THEN IMPROVE!!!