In my Kingmaker game, the PCs used Control Water on an army encamped near a river and it worked out quite well for them. Not a ton of casualties, but a lot of their camp and gear was wrecked, and since the attack happened in the middle of the night, morale was pretty ruined for all the soldiers in damp clothes.
For a 20th level guy, though? Geeze... Overwhelming Presence cast on anybody important seems good - it simply won't do to have your enemy's generals bowing profusely to the mighty wizard.
Alternately, Summon Monster IX may be for only 20 rounds (3 min 40 sec), but that's quite long enough for some creatures to win your army a battle decisively. Ice Devils have at-will Cone of Cold, Ice Storm, and Wall of Ice; Trumpet Archons can fly 90 ft per round and throw out DC19 paralysis saves for everyone within 100 ft; and Glabrezus have Reverse Gravity (14 10ft cubes) at will. Oh look, 3 of your most important units got lifted up 30 ft in the air; those make great archery targets, don't they? Or just make 1d4+1 Rocs and let them go to town for a few minutes.
Or, just, y'know, start sending out Demands...