ick! Earthquake

[Baalz]

Counters to earthquake:
Etherealness
high protection (16+)
high defense (16+)
high hit points
take out the casters before they can cast
rely on combat summons scripted to go after the earthquake (elementals, skellispam, etc).

Now, obviously many of those counters are gonna be hard to get on a bunch of units unless you're lucky enough to have the right stuff available nationally. What you'll want to do if you don't have those units is use fairly concentrated squads you can more easily buff. Forge some heavy armor for your mages (encumbrance is gonna suck but it beats dying), outfit some thugs with high defense/protection/etherealness, and generally don't field large armies of tasty targets.

In the realm of stopping him before he can cast it you've got two choices, you could try and assassinate if that's an option, or you can try to outguess where your opponent is moving and have a counter-squad there to meet him, getting the first turn drop. What you do with that first turn drop depends on what's avaible to you and what type of army he's got supporting his earthquake casters. Scripting anti-undead spells will all target that risin oracle if there are no other undead. Scripting flyers to attack large enemy monsters or rearmost could be a good choice depending on his army. Heck, just having a bunch of cheap mages cast body ethereal on some elephants first turn could work.