RTR 6.0 is ok, but it needs some serious outside mods to make it difficult in the least bit.
1) A morale mod is absolutely required. I personally settled for a mod somewhere between Wallaces super elite fight to the last man mod, and his tamer version. Ultimately people seem to route at around 70% casualties.
2) OTZ No Phalanx Mod. I was skeptical at first, took me a week to even look at the thread itself. Once I did no looking back. The problems with the Phalanx AI, and how the AI deals with the Phalanx, both with or against is horrible. The No Phalanx mod brings back some of the flavor of MTW battles. The AI is more adept at handling them. My campaign as Bactria was a tooth and nail struggle with seleucids for awhile. Unfortunately I encounter the AI turn freeze in RTR around 230 BC. I've since uninstalled and am waiting for EB or BI.