This is a pvp style encounter where your raid fights a team of 6 Champions (10 Champions in 25man mode) from the Alliance. Of the 6 Champions, 2 of which are Healers, and the other 4 are a mix of melee and ranged DPS classes.
Due to the nature of such encounters, there is no hard specific strategy involved. But the general idea is to nuke down the healers first, then followed by the DPS, starting from the most threatening ones (such as rogues and warriors) and eventually finishing up with the least threatening ones, such as a mage. From a priest's perspective, Rogues and Warriors are the most threatening since they can dish out healing debuffs and interrupts, which may lead to wipes. Many priest abilities that are not generally used in the PvE scene can really come in handy for this encounter, such as Psychic Scream and Fear Ward. Dispersion is also extremely handy as the opponents will employ CC tactics very often as well as mana being extremely intensive with the multi-dotting and the healing. Specific PVP talents, such as Improved Psychic Scream, Phantasm and Paralysis are helpful too, but not entirely necessary.
Overall the hardest part of the fight is right at the start when the first healer is being burned down. If anything bad happens during this time, such as being interrupted or CC'ed, it is pretty much a wipe. 4pc T13 is extremely useful for bursting down the first healer at the start.