I like a lot of the ideas mentioned by bretwalda .

The secret is to get a strong army up against the back edge so that you can rout units as they enter.

Since you are attacking you should be able to choose the army that you want to use.

The best way to get a strong army up against the back edge begins with the basic build strategy - I prefer cav archers as my first wave in the biggest battles - so I build lots of cav archers.

In VI one can organize easily the order in which the units appear. For the biggest battles I place a lot of cav archers - maybe up to 15 units - as the first wave. I use these to harass and deplete - but not rout - enemy units. Withdraw the cav archers en masse when their ammo is gone.

Second wave is mostly heavy inf with some cav for flanking. Slowly approach the depleted and hopefully tired enemy and hit em hard, as they rout chase with the cav. If the heavy inf is still intact follow with them. Withdraw any tired or depleted unit.

Third wave is heavy cav. Quickly bring the heavy cav third wave up to reinforce the heavy inf and cav on the far edge. If it looks like the enemy is going to be able to bring on more fresh troops than you can handle, fall back and regroup and try again.

The secret is not to chase with depleted tired units. By using the cav archers first you get the advantage (unless you start to rout enemy units then its a waste of time). By using heavy cav as a third wave you can quickly reinforce the second wave.

There's lots of variations and different situations, and I'm no expert, but I have found that using cav archers to soften up those huge armies is a nice solution to the problem you describe.

ichi