I always put the killer rune on him and prioritized sharpening his sword. His defense is high and his health is high, so he doesn't really need to attack from the back row. His damage output is also pretty solid too, so just giving him a huge crit% boost seemed like the choice to me.

Basically for me, Viktor just attacked, ate hits, and did nothing else. He's too slow to be an off healer, and he gets too few spells to handle support magic between attacks during the tougher bosses. Just having guaranteed damage every turn with a good shot at crits made him a high tier character for me, and in a party of 6 there's space for a guy who just hits stuff.