Well, I assume you're playing RTW 1.2 so...

In a big battle with the Marian Romans you have
1. Inferior - far inferior infantry.
2. Deadly Chariots - better go with archer ones
3. World-murdering Archers - Pharoah's bowmen
4. Ordinary, even slightly weak phalanxes - Pharoah's guards are pretty good, though
5. Cavalry (Cleruch) which matches Roman's after Reform ones. (What a cheat...) Though the Romans, in my experience, rarely use their Legionary/Praetorian Cavalry. They still have Generals cavalry, though.
6. Oh, and superb infantry flanker in the form of Axemen. You may or may not weaken them yourself already but they're still a great flanker nonetheless.

You will often face a massive infantry force that can easily beat yours...Urban Cohort!

Place your chariots on far left/right from your line.
Place your cavalry for flanking operation, as usual.
Place your Axemen close to the edge of your phalanx.
Use your phalanx as the main line. Place them on top of each other if you want to really pin down those Roman demi-gods.
Place your archers/onagers behind your phalanxes.
Like this...
C=chariot, c=cavalry, a=axemen, p=phalanx, A=archer/onager, -=empty

CC-----pppppp-----CC
--cc-aa-AAAA-aa-cc--

Use your chariots to disrupt enemy lines. Some Roman infantry will follow you; if not, they'll slowly be shot down freely. If they send cavalry against you (most likely) charge them and decimate them. Chariots own cavalry. It's likely that they'll lose their generals this way.

While they approach your phalanx, your archers will slaughter them. If you use onager use fire. No Urban Cohorts will survive if they are hit by burning stones. Burning arrows do you no good, however.

Stretch your cavalry far on your flank while they come.

When they reach your phalanx flank them with axemen, if your phalanx/axemen began to waver launch a cavalry charge on their rear; retreat after charge. Don't let them be bogged down. If the Roman line is thick you may even want to order your archers to shoot down those who're behind.

If they try to flank your axemen with spare Legions, rain arrows on those unit. Storm one unit at a time. They'll be decimated in no time. If not, charge your cavalry in, and out; on their rear/flank.

If you have spare chariots, let them shoot down Roman infantry on their rear. If their lines are thick enough (while fighting) you'll have minimal friendly fire.

The result should be you flanking them but they're thick in the middle and put pressure on your phalanx. When you feel like finishing them charge in your cavalry the last time. You should be sure that they'll break down by this rear-charge when you do.

After that it's mopping up.

P.S. If things get fishy you may even sacrifice some archers by engaging Romans on their open flanks/rear.