Be Prepared
If the party makes a bunch of noise on the way in, the summoning should be completed just as they arive. To get this effect, simply have an encounter outside the necromancer's room - plenty of warning.
Cover / Line of Sight
The necromancer should have easy access to cover, and the other monsters should provide barriers to line-of-sight.
Powers
If you're not opposed to adding powers to your elites/bosses, consider two types: defensive and healing.
A minor action to go invisible until next attack works well. Nothing like a -5 modifier to convince the party not waste daily/encounter powers on the boss.
Likewise, allowing the necromancer to banish a minion to provide temporary HP also works well - this way you get the benefit of the ignored monster transferred directly to the necromancer.
Snilvor, Goblin Emissaryddi (In the DM's Kit adventure)
Triggered Actions
"Die For Me" At-Will
Trigger: A minion ally within Snilvor’s line of sight drops to 0 hit points.
Effect (No Action): Snilvor gains 5 temporary hit points.
