Field smaller armies... I don't think I have ever had a 20 unit stack, I am way too cheap for that.

Try to limit the armies you field. Just coz you can afford 6 fullstacks of TAB's doesn't mean that its the best choice to wipe out 2 province Pontus....for example.

Make things intentionally difficult. Don't recruit units until you are under attack. Avoid gamey tactics. Change your tactics from the usual and try new stuff. Do the unexpected in your campaigns....I invaded the baltic and steppes as my first order of business in my sweboz game. THAT forced a real re-think of how I conduct war. Once I beat the Saka I might go and invade south to the Saba.

If this is still to easy limit yourself to 10 unit armies.....9 unit armies.... and so on. If you are so good that you can win the whole game using a single unit, not retrained, to conquer the entire map then maybe go try a different game. Until then there are ways to make the game harder.