"Väinö is a good wizard."
Translation:Väinö on hyvä velho.
No, it would be "Väinöllä on hyvä velho".
That was a good guess, though, because when talking about having something you generally use the partitive case except for a situation where you have only one of something. In that case you use the nominative case. If Väinö had more wizards, say 3, it would be "Väinöllä on 3 hyvää velhoa".
The partitive case alone without a number in front of it means "some". So "Väinöllä on hyvää velhoa" kinda sounds like Väinö is about to eat some wizard meat.
To clarify: for uncountable nouns you would use the partitive case. For countable nouns you use the nominative case "velho" when you have only one wizard and the partitive case "x velhoa" when you have an x number of wizards.