Bring in some javelins!!!
Forget the crossbows.
Cavalry don't matter too much in the bridge battle (but HA's can win the battle if you lose the bridge)
bring in 2-4 archers. The greeks have awesome professional archers. Set them to flaming arrows.
either let the enemy cross the bridge unopposed, or throw away one of your spear units so you can pepper them with arrows a little longer.
after the enemy crosses the bridge your javelins will win the battle for you. how?
set them up like this...
........|........... | <---bridge
........|........... |
........|........... |
____|EEEEEE|_____
... A..EEEEEEEEE..A <-----E= enemy soldiers
.....A... EEEEE... A <---A=3 units of acritae
......A............. A
........AAAAAAA
The trick is to set your javelins to guardmode so they keep shooting even if a few enemy units engage. HAve a couple Ippoakritai behind your javelins for even more javelin-throwing excitement.
To counter the persian horses bring in lots of Mourtatoi archers, javelins, and spearmen. When you set up your army Your archers will be the front line, but only se they can properly place the stakes. I find it better to form a Bulls horn style with stakes rather then a straight line. After battle begins your archers run to the back of your lines, and your javelins run to the archers old position. The spearmen will not be in the center of your army. They will be protecting your flanks. Put 2-handed axes in the center instead if you can.
note: this formation is too crazy to draw. I'm getting a screenshot, hold tight.
Here is the initial deployment phase with the archers in front, and the javelins directly behind them. As soon as the battle begins the archers will run into the back, and the javelins will take up their old position.
This is a side view of the same thing. You can see the javelin cavalry are already set up for a flank attack, and they are ready to counter the inevitable enemy flank. Also note how the spearmen are protecting the flanks rather then holding the front line. Your not just preparing for a flank attack, your expecting it.
Now that the battle has begun some shuffling is in order. The archers run to the back where they are protected from cavalry charges from all 4 directions. The javelins run to the front, and are set to guard mode. The varangian guard move up just a little bit. They have just enough space to get a charge bonus against anything that attacks the javelins.
As you can see the archers are Completely safe. Artillery can also be dropped in the center (i forgot about them). No cavalry unit will dare attack directly, so they will flank, but your spears are totally prepared, and thanks to your foot, and mounted javelin units on the sides they will take heavy losses. Your foot archers beat horse archers. The Ai also tends to bring their HA's too close to the javelins, where they get obliterated in seconeds.
A note: Don't let your javelins fall back. 1 on 1 they can beat any spear unit. Let them take any charges too. They are easy to replace. Do not let the enemy break your formation.
This formation Won me a heroic victory against two opposing armies( I used hungarian two-handed axes, since I didn't have varangian guard). The first army Sent infantry straight ahead, and had all the cavalry attempt a flank on the sides. The sides look vulnerable to a flank, but the enemy cavalry was decimated by javelins, while the spearmen finished the job. The infantry routed before engaging.
MY cavalry swooped in, and mopped up, and were now all prepared for a killer flank against the seconed army.