So assuming you're focusing on it, you can reach 19 Intelligence, plus let's say you stick on a +4 headband for 23. This +6 modifier gives a bonus spell of each level 3rd through 6th, and two bonus spells 1st through 2nd.
Here are your feats: Scribe Scroll and Craft Wondrous Item bonus, Spell Penetration, Spell Focus/Greater Spell Focus (necromancy), Improved Initiative (so he gets an extra turn before going down, if all goes well), Defensive Combat Training, Combat Casting, and Improved Familiar (an imp or a quasit).
Here are your spell picks once the undead army's built up:
>(schools not prepared: evocation, enchantment, divination. flexible with the barred schools)
1st (6+1 per day): Ray of enfeeblement in the bonus slot, mage armor, grease, feather fall, expeditious retreat, jump, protection from good
>2nd (6+1 per day): scare in the bonus slot, false life, mirror image, invisibility, see invisibility, protection from arrows, glitterdust
3rd (5+1 per day): vampiric touch in the bonus slot, vampiric touch, ray of exhaustion, dispel magic, dispel magic, protection from energy
>4th (4+1 per day): Enervation x5 on a bad day, otherwise enervation in the bonus slot, stone shape, mnemonic enhancer, stoneskin, summon monster IV
5th (3+1 per day): Symbol of pain in the bonus slot, waves of fatigue, overland flight, teleport
>6th (2+1 per day): Circle of death in the bonus slot, programmed image, wall of iron
Have a contingency spell set up to dispel magic on himself if he stops breathing (hint: he can choose to do that in case he's prevented from normal casting).
In lieu of hard-style equipment, go wild with wands, potions and scrolls of low-level buff spells at caster level 8 (but who makes them? have fun with that element).