Well, I wasn't in that game, but they used artillery weapons (catapults) and armies as their main weapons. They defended the islands and primarily razed cities on the mainland, so the AI had no easy targets. The goal of destroying 8 cities really plays to the human's skills and away from the AI's.
The key, though, is that they got massive numbers of VPs for destroying each civ. The destruction of Western Rome wasn't too hard and they got a ton of VPs (20,000 or something ridiculous) because of all the units WRome still had. They nibbled on the edges of the empire, taking full advantage of the 8 city rule without exploiting it (at least, not intentionally, they claim, and I believe them).
I'd recommend checking out the thread, especially the last page where T-hawk discusses some of this, probably more clearly than I am here.
Arathorn