I don't see why it should be difficult to beat the Epeirotes, in my Roman campaign I smashed their fullstack army around Taras, crossed to attack Ambrakia and beat up another halfstack that had 3 elephants in it, and that was pretty much the end of them, I just had to walk around a few more turns with a mop to clean up the mess he'd made of his cities.

Simply, just use your flanking infantry not to protect against a flanking attack, but use them to flank yourself. Take the offensive, so that his phalanxes and illyrians can't force you into a setpiece battle. Phalangitai should break pretty easily, and even pezhetairoi can't last long if the rest of the army is crumbling around them. If he has Illyrians on the flank, concentrate some hastati and slam the rear with equites (or hippeis) and they should pretty much disintegrate.