I've got a better idea. I use my first leader to go to the enemies continent and settle a city, hopefully near some strategic resources, or a strategic location. I them use the leader to rush build the FP, and I settle the surrounding area like a swarm of bees. Once you make your ring of cities, you're laughing, as you have fully productive soldier factories, at his front door...
However, this won't work if he has completely settled his continent. That's why I usually go for the GL wonder, and then send a settler or 2 (usually 3 - with spear\pikemen) to settle on his continent and get some temples established. By the time you are ready to attack, you have a very nice secure beach head, and if it works out with the leader, you have soldier factories all around him...