Well, to summarize, a slot of people have stated;

1 Practice. And yes, some of the battles take more time if you play with pausing to allow yourself time to see what all your units are doing.

2 Using group commands

3 keeping some units together (for support, less routing)

4 Keeping complementary unts working together (missile, spear, sword, cavalry)

5 Watching what the enemy does and responding appropriately

6 Using terrain as best you can