I assassinate the enemy general, via a suicide squad, in a brief skirmish before the real armies collide.
Sacking the QB eh?
In Europa Barbarorum, my method of dealing with enemy armies that I'm more or less at equal strength with depends, typically, on the country I'm playing as. For example, Rome has a very balanced army, if somewhat light on calvary. Using the traditional army of Two Hestatii, two Principes, two Triarii, backed with a horde of skirmishers, archers, or slingers, occasionally a few mercenary infantry. The infantry units fit in to a sort of rock-paper-scissors pattern, where what you use each unit for is basically immediately obvious, and thus tactical depth is easy to establish and organize on the fly. It's flexible.
With them, you structure your army such that you can swap out one unit for a better rested unit quickly, to keep your enemy engaged, and tired off balance, while your own forces remain fresh.
The Greeks are very different. Depending on how you expand, the units available to you will be very different, but the backbone of the army remains the Hoplite foot soldier, with a spear and large round shield, and heavy armor. They are heavy infantry, with very little variation. They are expensive, but extremely powerful used correctly, but massively inflexible, weak be surrounded, or charged with calvary. As far as conquering goes, it is fairly ineffective without the backup of a wide variety of units, found throughout Greece (Rhodian Slingers, Cretan archers,) Africa (Elephants), and Asia Minor. For real fun though, go in a really weird direction. Capture Massalia, abandon Greece, play as a Helleno-celtic state stretching through France and Spain.