Once, when using Darthmod, playing as bongland, I BTFO mainland France, then went to go clean up Corsica. I found that, for some reason, the AI had doomstacked millitia in Corsica. Anyway, my 4000 men - and I'll admit, they were pretty good troops, I had riflemen, dragoon guards, foot guards and 4 units of 9 pounder guns - attacked. I promptly had to then spend a battle, fending off a total of 18,000 millitia with my 4000. By the end, my artillery had lost a lot of men from being charged, my infantry was, at best 1/4 strength, as was my cavalry, my skirmishers did better, but all were out of ammo.
Still the most satisfying battle I've ever fought.