To make the AI more challenging in SP

[Rick L]

The AI can be terrifying at the beginning of a single player game, a patsy in mid game, and boring, why bother, in late game. To make the late game more challenging, play two strong units as human. When you get to the lat game, turn whichever is stronger over to the AI (the become computer controlled command in the options menu).

I just did this playing Helheim and Hinnom. I avoided all the "Hinnom eats everything" units by not taking any luck and managed to do okay against the AI. When I turned it over to the Ai, I had all the Lords of Civilization summoned. It made for a pretty challenging late game. Nethgul was the key to victory.