Let me clarify.
This is only relevant when playing with a Temple of Asyra opening and there sure are other ways to playing Priestess, especially if you want a quicker game.
That being said, it is a "guard-swarm", so its not meant for attack. Cleric vs. Bitterwood fox is an equal fight if you do the numbers. Having 5 clerics (25 mana) will give you 30 hitpoints of guards, two Knights of Westlock (26 mana) is only 20 hit points and are easier to focus fire (yes, they have armor and defense which is powerful, but both can be mitigated). If a knight dies you will lose tempo, if a cleric dies, it is not a big deal. One guard can be controlled, three guards is a bit more tricky. The clerics actually have the same number of attack dice as the two knights although it counts as less against armor. The trick is to keep the damage spread out, using other clerics to heal the damaged ones if able and generate mana when possible.
The whole point would be an alternative way to turtle-up untill you gain tempo. Once the clerics are out (around turn 3-4) you start summoning archers and angels (assuming you have your temples out). With 12-14 mana/turn you can summon one every turn and still have a bit to spare. Or you could cast Valshalla and watch her power up as the clerics die (because they eventually will). If your opponent instead focus on the new threat, your clerics become a free minor heal/turn.
This is of course speculation, but looking at the cleric I can only see them being good in larger numbers.