A common used strategy in antiquity, Scipio used it on the Battle of Zama, where he obtained a definitive victory over Hannibal after years of war, for example. It consists on sorrounding the enemy with cavalry to rearcharge enemy infantry, so cavalry superiority is needed to put this into practice.
We must choose infantry that can withstand a prolonged melee; generally any line of decent swordsmen or spearmen with shields should work. On the other hand, we don't need them to be able to defeat the enemy either, because they will be supported by cavalry, so everything we can save on infantry to invest on cavalry is welcome. As for cavalry, we need units with great charge bonus.
Here you have a video where this strategy is applied.
And an example of a big funds roman army for Rome II:
- 1x Legatus
- 2x Socii Equites
- 2x Socii Equites Extraordinarii
- 2x Equites
- 3x Auxiliary Sirian Archers
- 4x Hastati
- 4x Principes
- 2x Triarii