Lots of good ideas, thanks, it looks like there's a way to counter anything in Dominions, which is what makes it a great game
The best suggestion I saw was the cheap ethereal trick, dispossessed spirits. I think regular ethereal/high defense troops would be too expensive... LA Arcosephale elephants get three attacks per turn (trunk & 2 spear), so even without the trample I think they'd be strong against them.
How good are slingers against high-protection but shieldless troops? LA Elephants have strong armor too.
That's probably the best strategy -- lots of high-def troops up front to absorb the charge, lots of slingers/archers in the back to actually do damage.