There are many ways to resist necrotic, the most basic of which is the Wizard 10 Utility "Resistance" - at the level under discussion it will provide Resist 30+ to necrotic, making Orcus's Aura irrelevant.
In my opinion, the optimal character to pull this trick off is a multiclass elf wizard/cleric (tricked out Int and Wis for control style wizard powers, Divine Oracle paragon path). You get 3 attempts to make your Will attack (the last with a +2 bonus, and remember, you will likely have other bonuses to your attack from other PCs/sources), and you can have many utility powers to evade attack - the most basic of which should be the most excellent cloak of invisibility coupled with flight.
A flying, invisible Waczilla gets the job done.