Vel’Koz has a kit consisting of 4 skillshots making him very vulnerable to champions with dashes.
Abusing his slow moving ultimate with a dash or a blink into / behind Vel’Koz.
Save your heavy CC to interrupt his Lifeform Disintegration Ray. It hits heavily and can be the difference from winning and losing team fights.
A lot of Vel’Koz’ damage is applied by his passive which applies true damage. Thus stacking MR against him is less effective than against other magic damage casters. Try to wait out stacks applied beforehand before engaging.