Just out of curiosity how many units do you typically attack with? I found the Campaign AI is a bit too rushy due to recent changes to the conversion (now keeping forces together matters more) but if you defeat it's initial waves and dont immediately push back and rush yourself it tend to create a more stable front that sees larger forces clashing. Once you get that the RTS conversion is much better (although I still think grenadier/trooper conversions are too low) and you get decent starting forces.
I usually attack with 100% of max at once or a 50/50 split on two fronts. Depends on the map.
Had a very interesting experience today though. The AI on insane was building a LARGE quantity of warlocks. And it was using the special abilities really well. They annihilated like 4/5ths of my army just with mass spamming death from above. Quite nasty.
