I'd teach them some strategy instead. Make them go for the weaker points in the target empire. If there are heroes and large armies in city X, and only a tiny force in town Y, sneak around to Y and raid it. If town Y is important to the empire, that's even better. And, best of all... If the empire goes "OH SNAP" and responds by sending a force to try to take it back from you, then:
1. You'll be able to defend, and you chose the place at which it would occur (that particular town), putting you in a much stronger position than if you had just blindly charged their well-defended city X
2. They've just left their previously well-guarded city much more open to attack, if they sent troops from it. (If they sent them from somewhere else, well, that's a different weaker location then) So while the enemy army is marching to retake town Y, you can assault city X.
3. If you had spies in their empire, you could know what their armies are doing and when, so you have some idea of whether they're sending an army out - unless, of course, they have counter-agents feeding you false information.
Basically, apply some of Sun Tzu's the Art of War.
Of course, if all the civs had Sun Tzu running them, things might be kind of weird. But individual rulers could perhaps have a strategy skill - and could be targeted for assassination, especially so if they have a high strategy skill and are winning a war.
For megabeasts, maybe they should be able to use their non-megabeast worshippers to spy on towns to locate weak ones, and preferentially target those. Dragons perhaps ought to be able to just fly over towns and spy on them, too.