Fought against the Ptollies elite phalanx units on VH. They must be surrounded and attacked from the rear, preferably after being pelted with missiles (again from the rear). The objective of the phlanx is to pin the enemy, not to break it. Nothing likes being charged from the rear, take a look at the morale of the unit to assess if and when they'll break.

Best thing to do is unleash a load of javelins from behind, or better still arrows and slingshot. You can do this after they become engaged.