Depends what army you have available.

One thing that might work is to not attack them but click behind them and have your troops slowly (possibly in Guard Mode) walk through his formation. Then when in the middle of it, suddenly click on his troops to attack.

That is a typical MP cheesey tactic against dense phalanxes. You could also move Cretans or Balearics up to just in range of the units on the opposite side and shoot them in the back.

But then, I never play MP, ask Fluvius, I hear he is pretty good MP ;-)