)1. You can't win everywhere. Concentrate and focus on one spot and hold elsewhere. This probably applies to other eras also.
2. This era is definitely a 'combined arms' one. All variety of units complement the others and have their specific role.
3. Cavalry/horse. You must have superiority(or at least parity) to win a battle.
4. Pursuing horse will ride off into the sunset never to be seen again. I assume they've found the nearest pub/tavern or are rescuing some young maidens from their virginity.
5. Apply withering fire from your shot units to one unit at a time to break them. Gunpowder units seem to be particularly destructive.
6.Artillery. Find the nearest high ground and rain death on large/densely packed units.
7.Skirmishers particularly in woods can produce horrendous casualties on units that don't like to venture into woods.
8.Pike units. Never send them into any terrain that isn't steam-roller flat.
9.When things go bad, it seems to happen all of a sudden.
I'm sure there are more insights, but this is what I've gleaned so far. Feel free to add your insights and observances.
Cheers
Alex