Viktor got slightly overnerfed
The problem about him is litearlly his mana cost on his E early and mid game, he has to straight up rush damage after the 1st hexcore instead of the 2nd, if he does that he will lack damage, And its also essential part of his kit, if you played him in ranked before you will understand. He litearlly depletes his mana so easily and wont be able to keep up with the laning phase
My suggestion is COST: 70 / 80 / 90 / 100 / 110 mana to COST: 60 / 70 / 80 / 90 / 100 mana for his E
. If you are being greedy and using E every single time a wave comes in then yeah you deserve to go oom. You have to learn to conserve mana instead of mindlessly shoving wave after wave with 1 spell. Just as you said "he has to straight up rush damage after the 1st hexcore instead of the 2nd, if he does that he will lack damage" you can either pick going damage and having a low mana pool or getting extra mana and being able to spam more. You shouldn't have both.