Fighting phalanx

[fallen851]

My armies consist mainly of Camilian and Polibian hestai and principles with 2-3 units of Numebian skirmishers.

It sounds as if your army composition is the issue. Triarii, especially during the Camillian Era are key since you're having trouble against elites. Triarii are the best equipped Roman infantry to handle opposing elite troops early on. Also cavalry is key against a phalanx, allowing you to flank the phalanx quickly while your infantry to engage the phalanx from the front, and then your cavarly to hit the phalanx from the rear. If the phalanx turns to face your cavalry, then your infantry can slaughter them but try and stop your horses before they hit the wall of pikes. Also, most phalanx units are more vunerable to missiles than most other "civilized" infantry, so bring more missile units with you. They are most effective hitting a phalanx in the rear with missiles, but they are also effective after they run out of ammo because they are relatively quick infantry that can flank the slow pikemen and do a lot of damage by charging the phalanx from the rear and engaging in a melee battle.

If you insist on using the army composition that you have, change the formation your army is in to an asymmetrical formation.

The one I use all the time in multiplayer battles (Vanilla) is to refuse one flank and weight the other, seen in my diagram below (the 20 O's represent the enemy phalanx, the 20 X's are my short spear/sword infantry):

..OOOOOOOOOOOOOOOOOOOO


....XXXXX
..XXXXXXX
...............XXXX
........................XXXX

When my units meet the enemy line, my units in the center and right flank halt, so it looks like this in the battle:

..OOOOOOOOOOOOOOOOOOOO
....XXXXX
..XXXXXXX
...............XXXX
........................XXXX

Then the units in the 2nd line of my left flank use their superior numbers to push and flank my enemies right flank, and collapse that part of their line:
.OO
XXOOOOOOOOOOOOOOOOOO
XXXXXX
..XXXX
...............XXXX
........................XXXX

If my enemy attempts to help his right flank with units from the center or left flank (indicated by the blue arrow), this will expose these units and his line, as they must come out of their phalanx formation, allowing my center or right flank to charge and exploit the weakpoint created, and hit his phalanx units from the side:
.OO.....<-- OOO
..XXOOOOO..X..OOOOOOOOO
XXXXXX ......XXX
..XXXX
..........................XXXX


Finally, if the enemy advances on my shorter line in an attempt to overextend my right flank, their units will naturally break formation, and this will result in an ability for my units to easily hit the phalanx on the side where it is vulnerable (indicated by the red arrows) and rout the phalanx long before the enemy move their units to extreme right of my line and successfully flank me since my units on the right are so far back:

..OOOOOOO
....XXXXX->O
..XXXXXXX->OOO
.................XXXX->OOOOOOOOO
............................XXXX


This is the way to fight phalanxes when you must rely heavily on an infantry army.