To get phalanx u need to recruit pikemen (note: rome cant recruit pikemen unless u use mods)
How to use phalanx:
to active phalanx u need to click on your pike unit and click on the phalanx ability just like inspire
the phalanx slowly kills your enemies and push on
make sure that your phalanxs flanks are secured if not they will die easily
do not let your enemy attack u from the flanks nor the rear
since the main reason people use pike units because of there phalanxs
make sure all enemy archers and skirmishers are dead because pikemen are weak against arrows and javelins
its better if u put your pike unit where they cant be attacked for the rear nor the flanks
if you do all these correctly u should get kills.
Note: do not increase the pikemen speed while phalanx are active or they will unequip it
Thanks for reading my goofy ahhh guide good luck out there