The RathStrat guys typically do a Blitz 2-3 times a week.

From what they're saying...
You need one very geared tank, or two fairly geared tanks.
So they usually try to queue with 5+ PvE geared tanks, to ensure at least two get in.

Everyone focus fires one of the guys with Vann first (sorry, not sure which one).
And once he's dead, switch to Vann himself.

We don't need an especially large number of healers, for a Blitz.
In a typical RathStrat, we have 6-10 heals and I've seen successful Blitz's with six heals.