You need to know the correct order id to tell the unit to do to use the spell. So look at the spell you want the boss to cast and check whats in the "Text - Order String - Use/Turn On (aord)" field in the Object Editor. Then just order the hero to cast whatever spell is listed there with a trigger-- as long as no two abilities have the same order string it will cast exactly how you anticipate.
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Unit - Order (Last Created Unit) to Undead Necromancer - Unholy Frenzy (Triggering Unit)