After Son of Horus made a great tactic thread i thought i would offer another approach, the classic anvil and hammer.
The anvil unit its vital it needs to be strong, defensive and most importantly it musnt run away. A battle standard bearer is always a good choice for an anvil as is any stubborn, heavily armoured, or best of all unbreakable unit. The basic principle is the that the anvil unit moves up to block the enemy advance, once the enemy are locked in combat with the anvil the Hammer moves up to break the enemy against the anvil.
The Hammer unit should be fast and very good at killing quickly, a great place for your general. Good units for this roll should have high strtength and attacks, ranks and a standard are not as important for the hammer as the anvil provides those.
Both of the Main units need support the Hammer musnt get bogged down so fast cavalry and chariots are a good way to keep the enemy off there flanks.The anvil can struggle against greater numbers while it waits for the Hammer to charge, so firepower units are useful so thin enemy units down before they reach combat.
When timed well this can be a game winning tactic it can be used to take centre field from the enemy leaving them disorganised and often with their best units destroyed.
I hope this helps or inspires some of you (sorry about the poor picture it was a quick scribble on paint)