I just wanted to post this team build so it would be around for an easy to find reference.
http://wiki.guildwars.com/wiki/User:7hir7een/Builds/Build:Team_-_Triple_Necro_VanquishThis is the famous "Sabway" necro team build that folks use to do vanquishing in hard mode, and in general for PvE is a pretty much unstoppable team. Throw in a spirit spammer ritualist, some other killers and one protection monk (you almost never need two monks with this) and you should be able to beat just about anything.
Requires three necros (you and two heroes or three heroes) so you need Olias, Master of Whispers and Livia. Be sure when it comes time to choose between Master of Whispers and Margrid the Sly in Nightfall that you take the quest that gives you the necro. You can get tons of other ranger heroes.
