Your standard mix formation should look something like this:
Code:
Your forces
A A G A A
C C HI P P P P P P P HI C C
* * * * * * * * * * * * *
* * * * *
Enemy forces
Where
G= General
A= Archers
C = cav
HI = Heavy infantry or other elite forces
P = Phalanx or HI or mid-grade infantry.
If you deviate from the above (and there are many, many reasons why you should, in various circumstances) you should know why you are doing so.
The default strategy of many (including the AI) is to meet the enemy with front lines ('P' units) and then out flank them with your ends ('HI' in this case). Of course your enemy is trying to do the exact same thing to you. So, in these slug fests, it often comes down to troop quality and whose front line is longer; UNLESS you throw a monkey wrench into the works. Here are a few monkey wrenchs that I know work for 'Hard' and 'Very Hard' battle difficulty:
1.) Run your cav down the outside flanks of the enemy, as they approach. Usually, the outside units of the enemy's front line will stop and turn to address the possibility of a cav charge to their flank. This slows them down and can turn them so that they are more vulnerable to your archer fire. It also gets the enemy's nice straight front all out of whack, so that they hit your line piecemeal.
2.) Concentrate all your archer fire on a few selected enemy units, making them weaker and sapping their morale. This makes it easier to start an enemy rout.
3.) Use your cav to run the flanks (as in #1) and then dive in to charge selected 'soft' targets, like archers. If any enemy infantry turn around to get your cav, make sure that your archers fire into their backs.
4.) BEFORE battle, make sure your stacks position on the campaign map is such that you can only be attacked from downhill. Then, in the battle map, place your archers on as high ground as you can. The extra range your archers get, plus the shortened range the enemy gets, adds up to a big advantage for you. Also, the enemy needs to fight uphill to get to you, which is another big advantage for you.
5.) If you get good at 4, you will likely want to get more archers, so that the enemy is slaughtered before they can even close to melee.
6.) If the enemy is low on cav support, separate your cav and general out as a second force, which can sweep around behind the enemy and take out the general and less protected troops (e.g., archers). This forces the enemy to address two fronts, breaks up their line (so that your infantry can deal with it peicemeal) and lowers their morale.
7.) Hide your forces, except general, in the trees. Let the enemy chase your general a while, to tire them out, then run the general through your lines. As the tired enemy approaches, in disarray, pop your attack on them.
8.) Forget foot troops totally. Get your half stack of heavy cav (with 1 to 4 generals in the half-stack) attacked in open, hilly terrain. Run your cav from corner to corner of the map (Clock wise or CCW) and stop to rest in each corner, while the enemy runs/walks non-stop to catch you. After the entire enemy force is exhausted, start picking off stragglers, using 3 cav to simultaneously charge a single enemy unit from 3 different directions. With practice, you can consistently wipe out entire stacks of quality enemy troops, with 10% or less casualties to your half-stack.
9.) If playing as Gaul, get two Druid units to stand behind your lines. Set them to chanting and do NOT move them. Set your 4 units of upgraded Foresters to fire at will, and set your 8 units of Chosen Swords (your front line) to war cry, 10 yards before the enemy makes contact. Often the entire enemy stack will insta-rout before any of them make melee contact.
10.) stand your stack on a bridge, and let them get attacked. Make sure your stack has at least four (4) quality archer units. On the battle map, plug the end of the bridge with a unit, or three units in a 'U' formation. Pack you other foot soldiers behind the 'U'. Set your archers up on your left flank, so that they shoot into the enemys' unshielded right side as they cross the bridge. Enjoy the turkey shoot.
There are more: dig around in this thread, and see what interests you.
I hope that helped.