Here's where the important decision is. If you really want to destroy Epieros, go ahead and take Serdike and Taras. However, I will warn you that Taras might be a poor decision. The reason why is that Taras has a wonderful barracks, but it can't be used by the Makedonian faction, just Koinon Hellenon and Epeiros. That is why as Koinon Hellenon, it is advisable to take Taras after the Romani conquer it so that they can train powerful units immediately after occupation. You as the Makedonians however, will have a terrible time should the Romans attack (which they probably will). It would take years for your MICs to be built to a substantial level in order for retraining to occur, and the onslaught of Pedites Extraordinarii, Triarii, and Hastati may overwhelm your initial forces.

However, if you have a capable phalanx-based army and you use your heavy cavalry in coordination with hammer-and-anvil tactics, you should be able to counter the Romans if for a year or two if you have enough soldiers to hold them off. The AI is terrible with dealing with phalanxes, and even Epeirote cavalry can destroy the Roman bodyguard cavalry. So it is easy to charge them from behind to route the Romans (I'm used to playing on VH/VH).

If I wanted to truly cripple the Epeirotes, I would take Serdike instead and work on rebuilding your homeland and preparing to assault Rome. The Romans will become a powerful adversary soon, and they are a mess to deal with should you decide to fight them. There is a good chance the Romans will lay siege to Taras anyway and conquer the Epeirotes there for you. So if I were you, I would simply take out the Epeirotes in Serdike and wait for the computer to do the rest.

The choice is yours. Hail Strategos!