How to easily play viktor:
Just bully your lane and farm well. Make sure to augment your E as one of your first items. Your augmented E is your bread and butter as for damage and waveclear. Try to land your W when someone else in your team has a stun/snare/root/whatever. That way they will stay in your W instead of just walking out of it, so they get stunned instead of slowed. In teamfights, use your ult pretty early on. The AOE damage it provides is pretty underestimated.
The CDR for Viktor is just not as effective as building straight AP currently. With Luden's being dependend on total AP and Rabadons giving 30% more AP ap is just such a sweet. Current items that bring CDR are also not really needed on viktor, but in special cases.

Issue is that most CDR items have mana regen, which is not really needed on viktor (once you manage that mana) I mostly go for 2 Dorans in lane which is more than enough to keep harrassing my enemy + pushing the lane. Buying Morello or Athenes is not worth it at this point unless they have vlad (-> morello) / super high ap burst or ap top + mid (-> athenes) as you get so much more from pure ap
I tend to try to stay in lane until i have 1100 gold, or more, on my first back and upgrade my E along with buying boots, pots and/or wards depending on your back timer. Once you upgrade your E, with enough AP you should be able to E and entire creep wave and clear the entire back line. Also with upgraded E you can try to place it on the enemy wraithes/raptors and kill the babies over the wall. Make sure you know the jungler is not there and you can't get collapsed on. After that I tend to try to back with around 1325 gold, or more, and upgrade my Q. After that I just go with the main items I need. Abyssal if vs heavy AP, Morello into LichBane or just sraight DCap/Ludens Void Staff. If it gets to late game and I have 1000 gold I can spend then I get the 3rd upgrade in the Hex Core. I like to delay the 3rd upgrade BUT MAKE SURE THAT YOU GET UPGRADED TIER 3 BOOTS, UPGRADED TRINKETS OR AN ELIXER BEFORE UPGRADING YOUR W. I tend to feel comfortably with upgrading my yellow trinket whenever I have the 250 g. Pro's tend to go with red trinket on mid laners but in my Plat Elo I value the vision more at around lvl 9-11