Speed is king in RTW (along with charge bonuses.) Cav have both of those in spades. Any proper use of cav will flank the phalanx and the phalanx units will collapse in seconds. Frontal attack can be used to pin the phalanx so that another cav can crush it. It is not the preferred method, but it works. After a few hoplites are gone, the rest are easy to contend with.

Against a cav army, the phalanx army can only circle its wagons. If they bring some cav, the cav army can still crush it so only the phalanx portion is left to contend with. This makes it a defensive army, that cannot attack, because it will lose by sitting there in defense.

Did I mention that hoplites are easy prey for missile units? Balearics will chew them up rapidly.