You can't destroy magical sites, ever. You might be able to preach them out, but with Marignon that's usually a bit tricky as well... of course, this all assumes they aren't using Diabolical Faith.
Abysia shares fire, astral and strong holy, but Anathemant Dragons are very expensive and their mages tend to be a bit on the imprecise side, and lack the national access to crossbows (flaming crossbow bolts are not good for undead!).