I'd just use my pikemen and laugh at their puny spears. Oh wait, you're Roman....

Use cavalry to single out archers and peltasts. Since the hoplites are so slow in a phalanx, it's an easy task to seperate and destroy their support troops away from the hoplites. And as mentioned before, onagers can really do some damage. Make sure to use firepots and keep your own men away! Firepots are just as inaccurate as they are lethal.

The most dangerous hoplites are armoured ones. Those guys are pretty damn arrow proof, so you'll have to eventually melee with them. Normal hoplites will quickly fall against a few units of archers. Horse archers are ideal, but they aren't exactly easy to recruit as mercs. The hoplites you cannot kill with arrows you'll have to finish off in a melee. Hoplites are slow, and the AI usually fails to maintain a decent battle line on the move. So you should be able to mug their hoplites one at a time with your infantry. The key to this is to get the Greeks to attack you first... otherwise they'll be sitting in their nice little defensive line. And then your flanking options are greatly reduced.

The Greeks can be pretty hard at times, but they just aren't as flexible (and thus dangerous) as many other factions.