How do I "fight the enemy where he isn't"?

It's basically another way of saying that you should attack the enemy in a place where his attention is not focused. As an example, say you have an enemy's fort under siege. Your enemy will be heavily focused on defending that fort, and may therefore hold back troops that would have otherwise been sent to defend his supply lines. So, you send a unit to wipe out or hold up those supply lines, and his fort will eventually run out of food and weapons.