I'm totally opposed to the people that say stretching your line to match the opponents line but that is there style.

My reccomendation is a deep formation (note must be on normal or huge unit sizes) You ca'nt really toy with depth with anything smaller.

The comp always stretches there lines for all factions as far as I can tell, although they do seem to give some depth for phalanx units regardless if it is full or not.

All I do when facing a stretched army is box all units up, makes them very maneuverable and able to absorb a charge. Now that I've been playing for a good bit I have yet to have a unit route form getting charged while in deep formation.

Heres what I do I box all my units up and if its a long enemy line I reduce the reserves. I advance the units into javelin throwing range unless they have misslesuperioty wich provokes me into an immediate charge. Also put them in guard mode so that units do'nt advance ahead of your line. Once you've depleted your javelins or have decided it's time to charge advacne close to the unit and then charge it. The reason for not charging at a unit from a distance is they aim for the center of the unit so you could have 2 units bunched together and would make big gaps in your line then again when the enemy is stretched they do'nt have reserves.

So heres what happens when the A.I does this they have a long line that is 3 deep and maybe cavalry in reserve. I gut there center quickly because 10 deep will quickly break up and punch through 3 deep units. If cavalry does charge in they will not have enough power to break up a 10 deep formation it just prolongs winning the center. Once the center is won your free to flank out. And since the units are coming in 1 at a time from the flanks they are pretty much piecmeal. Even now that I'm using the kill rate mod I still win the center quick enough before I have a barbarian horde flooding my flanks.