Rain of Fire, at DC 50, should kill most ground troops. It hits a two-mile radius, doing 1 hp/round (for 20 hours). How long does it take ground troops to run two miles? Twelve minutes? That's 120 rounds -- and thus 120 hp damage. And the survivors have probably left behind equipment and supplies.
If we have multiple epic spellcasters, we can probably hit them with a Rain of Fire and a Rain of Acid at the same time, doing 2 hp/round, and getting around resistance to just fire or just acid.
I'm not sure how we defend ourselves against the same thing. A Rain of Holy Water would sting.
Anyway, if the troops manage to stay together and progress in any one direction, we can just hit the area in front of them with another Rain of Fire. If they disperse in multiple different directions, that's when we mop them up with our "cavalry" -- teleported-in Fire Giants.