I've never used the Phalanx in huge numbers, since I've only played romans sof far. but I've always used them in the center to defend my archers (one or two are enough).

The Phalanx are fairly difficult for offense in an open field. When switching phalanx mode on/off, don't do it while paused, (don't do the run/walk toggle either while paused). The controls are not very reliable.

For medium-range skirmishers, they are specialized troops but really effective on slow units. When the AI has no cavalry, they are even better. I just drive them upfront, in the flank really quick while my army marches forward slowly.

When the AI takes the bait, I halt my whole army and let the archers do their work. If they continue to chase, I just block them with a unit then flank with a cavalry.

Against other skirmishers though, turn OFF the "skirmish mode" so they won't go crazy going back and forth. And press "hold" so they stand their ground

Also when they are behind your line, target the enemy farthest from you (disable fire-at-will mode).