___Be Prepared___

If the party makes a bunch of noise on the way in, the summoning should be completed just as they arise. 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 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 to go invisible until next attack works pretty good. Nothing like a -5 modifier to have the party not waste dailies/encounters on the boss.

Likewise, allowing the necromancer to banish a minion to provide temporary HP also works well - that way you get the benefit of the ignored monster transferred to the necromancer.

> Snilvor, Goblin Emissary<sup>[ddi][1]</sup> (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.


  [1]: http://www.wizards.com/dndinsider/compendium/monster.aspx?id=6035