| I would recommend at least two levels of Archmage. The first for Spell Power (your spells are as only good as their DC), and Spell Shaping (so you don't hit your minions). Remember to also take the feats Spell Focus and Greater Spell Focus. A 20th level with a 30 INT should then have a base DC of a 24 for a 1st level spell (base 10, +10 int, +2 for Greater Spell Focus, and +2 for Spell Power).
Although he may be a necromancer (and if the party knows he's a necromancer), he might want to have a good backup school just in case... | 
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