There are several guides online on how to win the various historical battles.
The Battle of Brandywine Creek (1777)
Lets Play - Empire Total War (DM) - Historic Battles - Brandywine Creek.
The Battle of Rossbach (1757)
Empire Total War - Battle of Rossbach
The Battle of Lagos (1759)
Empire Total War - Battle Of Lagos
The Battle of Fontenoy (1745)
Empire Total War - Battle Of Fontenoy
The Battle of Porto Novo (1759)
Empire Total War - Battle Of Porto Novo part 1
Apparently there are also mods for battles, as the first video shows the Battle of Poltava (1709) and the Battle of Lesnaya (1709).