In terms of army composition:

a nice mixture please (you never know what you're going to be facing in the field.)

So (if available): 2/4 units spears or pike; 2 archers; 2 x-bows or musket; 2 light cavalry; 2 missile cavalry; 2/4 heavy cavalry; 4/8 heavy infantry; general's unit; a couple of artillery pieces and whatever specialist units I take a fancy to.

always merge damaged units and send the remnants back to be retrained; rarely use mercs except if attacking a settlement or castle and then I demob them once the garrison doesn't need them.

On the battlefield, once I have every one lined up I assigned units to attack the enemy units (unless I'm adopting a purely defensive posture.) I let them finish the job, reform and then send them off to their next target.