I can say that in my current game on the Hardest/Hardest setting as the Scippii I have met a reasonable (but not insurmountable) challenge.
Battle Strategy and where I am now:
I easily took Syracuse within the first 4-5 turns, and then the Senate asked me to take Thurmon way over in Greece. I decided heck, why not, and did just that (normally I would have focused on Carthage). Meanwhile, my Navy was pounded by Carthage. From Thurmon I could wage war against Macedon, or Greece. I decided Corinth looked like a sweet prize (especially considering the Wonder there), and took it from Macedon, however I have since been constantly attacked by the Greeks, while watching the Macedonians grow in strength in Athens and Larissa. I feel like my Gaius Scippio is all alone in Greece with a dwindling army (I had about 600-700 men and now I am down to 4 unit cards of Hastati and my general). The constant siege/attack has finally ended, and I am hoping I can build up a force there until I can send re-inforcements from Sicily. Meanwhile, during the siege of Corinth, I took Lilybaum from Carthage, and began rebuilding my navy with Triremes instead of Biremes now that some of my ports had shipwrights.
I need to secure my foothold in Greece (Corinth), while bulking up my Navy to begin my work on the Carthaginians. Basically I have a two front war... Greece, which is a long supply line from my home in southern Italy and Sicily, and Carthage.
I also have several diplomats wandering the map... one way up in Germannia, and one that just reached Egypt.
Does anyone else have experience playing HARDEST/HARDEST? What are your experiences?