Well when it comes to phalanx, the best strategy really is to stop the enemy in place with your phalanx and wheel your cavalry around and flank the enemy. Now, if you're worried that your cavalry is inferior than the other I think I have an idea for you. If you are attacking or them, you could use a couple units of archers to concentrate on their cavalry units. Even if you don't totally destroy them (which you could do very well), you will leave them down in numbers, allowing your cavalry to overpower them.

Now, if that doesn't work, the only other idea I'd have is to buy some good Thracian mercenaries. This is kind of hard to give you information since I have the RTR mod, which has excellent heavy cavalry for the Macedonians.

Hope it helped!