I have found this to be a pain but found out an easy way to get this. To unlock the achievement you need to see it under the achievement area, but you can do it before and when it pops up in the achievement area it will be ready to unlock.
Ok so first appoint a General with a great person. Then go into their stats. You want to use EXP books (yes in game currency buys these). You start with about 5k in-game currency (blue Diamonds) and you need to spend about 1-1.5k. Just keep buying the Exp books. The other way to get this is basically spy on people and attack with an army about the same size. I have battled many people and kinked around with this but it seriously is a pain unless you get the Exp books and rank him to 10 using the in-game currency you get.
Good luck and hope this helps.