* Play with Historical Attrition ON. Then you get Replacements at Depots only. This is more realistic, IMHO, and forces invaders to retreat to a Depot point after a bloody repulse. IOW, ya gotta plan more & make darn sure of your Supplies, etc.
I second your recommendation to play with historical attrition, not only is it more realistic, counter-intuitively, it actually makes things easier against Athena:
It is very hard to completely destroy large formations in AGE games (well, enemy formations, it is easy to lose your own). Making Athena march her low cohesion, just defeated troops three or four regions back to the nearest depot to regain hits means you have a chance to follow up on your victories as they flee, and also to let weather and terrain do some (if not most) of the work for you. She typically won't move into bad positions intentionally, but the withdrawal subroutine often leaves defeated forces in terrible spots: combine bad weather, poor supply and poor terrain, and a withdrawal from a minor battle can turn into a Division losing catastrophe. The AI handles this situation poorly compared to a human, and it is relatively straight forward to then keep her bottled up in bad land for a turn or two until you can bring your fully recovered main force back to finish her off; with each blocking force she has to fight through to get home (even if she wins), the more hits she accumulates and the weaker her force is for the next engagement. (Play WIA for a crash course in this technique; the wilderness is your friend!) If your attrition settings were too "easy" she could simply rest a turn and come right back at you, or easily escape to fight another day (a bit of an oversimplification, but you get the idea). Without historical attrition she can also do things like lay siege in the winter, march divisions over the Appalachians safely or bypass well-defended depots while invading.
Historical attrition also clarifies and simplifies your strategic and tactical thinking. Depots become much more valuable, so removing/capturing/defending them become your obvious strategic goals. Tactically, you base operations around a secure, rail-connected depot in an area you are contesting, regularly rotating the short distance back to your base when you suffer too many casualties, ideally by rail. Distance to the nearest depot becomes a major tactical consideration, and the human can finesse this better than Athena. With low attrition settings, her forces are instead free to wander around the map, recovering hits pretty much wherever they happen to be and rarely having to leave the front lines to recover.