You choose a creature type as Witch's Vengeance resolves, not as you cast it. No player may take actions between the time you choose a creature type and the time those creatures get -3/-3.

Witch's Vengeance affects only creatures of the chosen type that are on the battlefield at the time it resolves. This includes creatures you control, creatures with hexproof, and creatures with protection from black. Creatures that enter the battlefield or become the chosen type later in the turn won't get -3/-3.

To choose a creature type, you must choose an existing creature type, such as Human or Knight. You can't choose multiple creature types, such as “Human Knight,” but choosing “Human” is enough to affect a Human Knight. Card types such as artifact can't be chosen; nor can supertypes such as legendary; nor can subtypes that aren't creature types, such as Vehicle.