The easiest way by far to gain favor is by getting the simple achievements. It's true you get no regular favor (Most Kills, Most Upgrades etc) from games against only the AI, but you DO get favor for any achievements you earn.
Example: Make a custom game on Prison against one Easy AI, say Queen of Thorns. Adjust the death penalty to light (so the AI spends less time dead) the xp gain rate to high and the respawn rate to high (more xp, more creeps) and I use conquest with the war rank starting at 10, so you can quickly end the game once you have your achievements.
Say you pick Beast, you get 50 favor for winning if you haven't won as him yet. 200 favor for Corpus Explodicus, which you'll get extremely easily after taking Post Mortem at level 5. Slimer is 100, I think, for doing 20k in damage in a single game with Ooze. Take ooze from the start and with the fast respawn throwing creeps at you should be no problem. Beast Almighty is another 100 for getting to level 20, high xp, lots of creeps, Easy fast respawning AI, won't take long. As soon as you hit 20 and have Slimer done, buy giants and finish it quick. That's 450 favor in a game that took me 15 minutes to complete.
Most of the Demigods have a handful of easy achievements you can get in a single game, and if not you can always jsut start a custom game at level 20 for an instant 100 favor. Do that for each of the Other demigods after playing my example beast game and you're at 1150, halfway to the 2k you need.
