Viktor is a strong, but still not really difficult to play ap champ. He can stun an entire team, silence it and deal a lot of damage to all enemys.

His passive gives you an item that gives 3 ap per lvl. You can buy 3 different add-ons in the shop.
I always get the recommended add-on. It gives extra damage on his e-spell and 45 extra ap.

I choose this rune setup, because of its high early game ap and its also very good in end game.
Of course you can change the Glyphs to ap per lvl.

My itembuild allows you to finish a whole team in a few seconds. With your high ap and the slow from rylais you can take down almost everyone easily.
If the early game isnt running good you should change the Mejai´s to a Rod of Ages.

I took this skill order, because you wont get much damage in the beginning and as soon as your lvl 2 your enemie has almost lost. You use the q-spell on the chamo or a near creep and then use the e-spell on him. With the shield of your q-spell he wont deal damage to you.
You shoul try to stun your enemies with the w-spell to make them stay in your ultimate as long sas possible.