I group my whole army as one and when I need to make a smaller group they need a new formation order (ie shift 1 single line) to break the link to the old big group formation.

Kyodai, as for tactics it seems flanking is everything especially with cav.Also cav shouldnt be left in melee too long pull out and recharge as often as you can and units rout pretty quick.

Missiles are far more effective firing at a flank (preferably the enemies right as this is unshielded) or rear.

Hope this helps a little.