This is not tragic as it sounds. All you need is a partner.
Lets break it into steps.
1) Create a multiplayer campaign! Go to the main menu, go to the first option (eagle like picture) and select the multiplayer campaign option. Add a password and inform your partner where to go and join.
2) Select Head to Head option, auto resolve, and set the difficulty to the highest (scroll your faction icon to the hardest difficulty)
3) Now i would suggest that you both select a faction next to each other, so you can attack right away without having problems from the AI. So the most ideal if you have the DLC's is Sparta and Athens.
4)Start the game
5) The Attacker should buy all available mercenaries, and march to the enemy base and then hit turn
6) The defender should disband all their units and leave the generals only (you can't disband them) and then hit turn.
7) The attacker should check one more time for mercenaries, attack and capture the defender's settlement, if you dint kill all his Generals completely, hit turn.
8) The defender should attack the attacker to "reclaim" his settlement and lose.
9) Achievement unlocked plus all other difficulties if you haven't got them so far, plus the faction achievement if you haven't won it already.
If you don't have a partner, message me, and we will arrange a day to help you.
Good luck :)