Play VH/M. VH battles do not improve the AI battle tactics, it just gives all their units insane morale boosts, where even a relatively low grade enemy unit can rout your elites.

Enemy elites, like the Seleucid generals bodyguards become nearly unstoppable monsters.

The only early unit you have that can put a dent in their armies are hordes of slingers. And they require a lot of micromanagement to use properly.