Khan VS Kahn

Each side must have at least 14 characters for a total population of 28. I found this easiest to do in couch mode, so you can control both sides. Just keep on top of your food supply, and it should take under 10 minutes.

The image for the achievement seems to indicate that you need to battle all 3 levels of AI (in single player).

but i only received the achievement after I won a single player battle against every race.

For winning against the hard races (elf and trolls).

(I won with elves mainly)
First round, build a woodcutters and choose the wood in the shrine.
Priorities using the woodcutter first. The AI won't be able to build much as you will collect all the wood.
Built up the tower as fast as you can (all you need is wood).
Keep your population close to the AIs (if they build a tavern).

If they build a farm, if it gets to 3 food, then take it from them. (Since you are elves, you don't need to hunt as often).