I usually put my skirmishers and/or archers in the first line, to fire off a volley or two and keep the enemy busy for a little bit before retreating behind the mainline troops and dispersing to each flank for envelopment and firing into their backs, accompanied by a small cavalry force to ram in and sweep the line when the enemy flank starts to waver.

Putting the missile troops in front entices the AI to focus troops on them, especially cavalry, so the cavalry can be disposed of quickly and the enemy becomes narrower and easier to envelop.