2. I always use the unit with the highest defense to pin the enemy from the front, and flank with whatever has the highest attack, highest lethality, and ap ability...In my KH campaign, for example, I'd hold with heavy inf (usually hoplitai) and flank with Rhomphaiaphoroi. The hoplites can hold all day and the Rhomphaiaphoroi will cut the enemy flanks to pieces.

If you don't have two handed units with high attack, lethality, and ap rating, I find it best to use light, javelin armed infantry (like peltasti) as flankers. A few rounds of javelins up the arse will rout just about anything. In addition, peltasts are also fair in melee. Thracian peltasts are especially good for this duty as they have ap melee weapons.