Just keep an eye on where Malz' DoT is, the purple shade on units. It jumps to other units on death and refunds some mana to Malz. It's also Malz main source of damage early on. His Q/silence is easy enough to dodge. Just takes some practice. The zone is simple, try not to stand in it when it's on the ground. Once he gets his ult, try not to get caught out of place. He can easily punish you since he can get easy damage on you if he hits you with the DoT, places his zone on you, then tethers you in them both with ult(also he can ignite you if he's in range during his ult).
Also never leave his Voidlings alone for too long. They can do a LOT of damage if you just ignore them, specially since it targets enemies marked with the Dot or tethered by the ult. Just try to keep track of how many spells Malz has used since the last one(Voidling spawns on every 4th spell, I believe) to try and know when to anticipate it and kill it before it gets stronger after 7 seconds being alive.
As Viktor, just never get close too him and try not to stand near any of your minions that have the DoT and are low. Try not to get peeled by it. I can see it as a somewhat easy match up since you can out range him with E and zone him away with R. Also don't forget about your W. Malz has no mobility aside from summoner's or any MS item they might build. If you feel you got too close and you're going to get ult'd, drop your W on him(or the edge of it on him with the center of the trap being between you). If he ults you, he'll just get stunned and would have wasted it or will run away and keep you safe from a potential death.
Also keep in mind that Malz can help out in jungle ganks a lot. All it takes is him to Flash + R to suppress you as the jungler arrives then takes you down so make sure to ward.