Cornering the AI

[Maloncanth]

After the first sixteen units on each side, additional units start mattering less. If you've cornered the AI with some big stacks in their last province, just arrange sixteen buff units with a few cavalry reinforcements and concentrate on winning the first leg of the battle. If you can kill/rout their general and deal severe damage (especially easy if the AI decides to try using siege engines against you) most likely their defense will fall apart and you can eat a lot of the rest of their stack even when it looks very impressive on paper.

Be sure to bring light cavalry though, to prevent rallies, especially if you happen to be playing XL with all the buffed up morale.

If you've cornered them into a corridor (for example, cornering the Russians in Finland while you hold Novgorod) then just leave a stack of eighteen or so and arrange a massacre defense.