After killing all the trash, take down the gnolls in the druid ring. Then kill the large encounter of gnolls inside the ring. After that, the High Priest will summon an elemental to attack you (random; earth, fire, water or air).
Immediately after killing the elemental the High Priest becomes attackable. He doesn't have any special tactics, but will spawn about 2-3 Heroic mobs every twenty seconds or so.
A good strategy is to have everyone focus on taking down the main boss as fast as possible, with the tanks sending out AoEs to bring down the adds. It is best to clump together so no one is sitting with an add out of range of the tank's AoEs.
High Shaman Vascha is a healer, most likely a defiler. He is mostly (~75%) crushing melee, while using a couple disease spells (damage and debuff).