Combat system
Spend some time learning the combat system. Even though it is simple, try to pay attention to what adjustments and shifts are happening in each combat. Being able to use the system to your advantage will prove invaluable, especially against live opponents.

Keep reserves
Keep plenty of reserves ready to exploit any holes you punch in enemy lines. Let the lead units do the dirty work, then push the fresh troops through the gap to wreak havoc on any remaining enemy units.

Retreat
Be sure to surround enemy units whenever possible so that a "retreat" result will annihilate those units.