WoW what an epic battle....but then...

[Taqwus]

Astral shield can be beat via high magic resistance, or by using non-melee attack forms.
Communicants can be killed through a variety of ways, depending on how protected they are -- somebody using Communion could take advantage of the "masters boost themselves along with communicants" feature and cast Invulnerability, Iron Will / Antimagic, Mistform, Elemental Fortitude, Astral Shield et al. If they're missing an elemental immunity, you could use something like Grip of Winter or Heat from Hell, or alternately Foul Vapours -- spells that mess the whole battlefield. There are non-elemental battlefield hitters that test only MR, like Astral Tempest and Soul Drain. Then there are methods that only test prot (like Rain of Stones), and methods that are basically unresistable (prot doesn't matter that much when you've just been smacked for 150 damage by a meteorite; Mage Bane is an unresistable KO as well). And of course, you can smack them outside of a set-piece battle -- Earth Attack, Leprosy, Fires from Afar, Flames from the Sky, Murdering Winter et al...