1. drown them in pilae
2. phalanxes are the masters of siege defense. try to wait sieges out or attack nearby field armies to get them on the battlefield.
3. kill gens early in the battles if u can
4. phalanxes will gouge holes in ur lines fairly quickly, so quick, decisive hammer-and-anvil tactics on ur part are essential.
5. Pre-marian cav sucks, while Macedon has some of the best cav in the game, so u'll need to get creative
Remember: the campaign isnt all about battles. Proper campaign strategy is essential.
Legio_Italica's Tried and True System to
Defeat a Strong Enemy:
1. Convert to a defensive war on all other enemies and focus on the one.
2. Blockade all ports, thus cutting the enemy's income and the morale of his citizens.
3. Send spies and asassins (diseased if possible) to wreak havoc on the enemy's interior
4. Let the effects of this soak in. Dont invade until you are absolutely ready.
5. Once you attack, blitz your enemy. Never allow him any respite. You are attacking, so maintain the initiative. Fight on your terms and only your terms. Anything else spells defeat or Pyrrhic victory.
6. Utterly destroy your enemy.

Leave no trace of him anywhere in the world; wipe him from the history books to ensure he never rises again to haunt you.