There are two questions here - is fighting uber-AS fun, interesting, and worth doing? And, if yes, how does one do it?

The responses thus far have more or less covered the latter, though given the AI's tactical weakness I'm quite sure that a Roman player could beat them in a head-on slugfest in the Egyptian desert (or Asia Minor, or both). The blitz raid strategy would be more interesting and effective, though. Unless you're using the BI exe, Crete and Cyprus should be safe staging areas (my generals seem to get "confined to bed" after just a couple of turns at sea - very annoying, I don't advise a nonstop Tarentum to Antioch voyage).

The important question is whether one can have an interesting game after the Gray Death gets going like this. As bovi indicated, the money scripts can help, and I think most are save-game compatible (from what I've read, the mercenary scripts in the balacing mods require a new game). One can also try to stir up trouble by sticking a spy in AS cities with low public order, and optionally having an assassin stand by to knock down temples or other buildings. Against a really large empire, you should certainly be able to flip a few cities around the edges, either to the rebels or to a nearby faction (Halicarnassus to KH, in one example I've done). Also, does AS really have the east locked up? At least in my game Bactria took a while to get going, but by the 230s they had absolutely torn apart the eastern third of AS. Anyway, lots of factors to consider, and it's probably worth toggling fog of war off for a bit to get a better sense of whether the game is likely to remain interesting (unless you already have accurate, current info on what's going on back east and in northern Asia Minor).