I'm going through the same thing. You need a decent sized empire, and that may mean earlier wars. You will probably need to do this in the Ancient age, because you will fall behind in tech in the Middle ages and the AI defenders will be stronger than your attackers. By the way - this is what I've learned I
should have done, not what I did do
It's even harder to do this if you're playing archipelago.
The one strategy I've been able to manage ok so far is picking the right tech to research, and Literature has worked for me because the AI doesn't seem to research it until late and so it has good trade value. The early techs are useless in trade value unless you can contact other civs very very early, and I haven't been able to research Writing by myself before anyone else gets it, which makes the Philosophy gambit hard to pull off.
You need to keep a decent defensive military at all times....partly to ward off the AI (you can't afford to lose land) and partly to stave off flips, since you'll probably be miles behind in culture.
Bottom line is keep your head above water as long as you can and check diplomatic screens almost every turn because smart trading is the only way you're going to keep in touch with the tech leaders.
All in all, a lot harder than Emperor.