First of all, this is the hardest mission in the normal game (not counting the crete dlc), its the last mission in the COOP ONLY series, so its doable only in coop (5th mission, after "we're like peas and carrots"). The enemy has 2 cities and they will atack in the first 5 minutes, and then continuenly attack forever with all type of units from infantry, range, cavalry to ballistas, rams and palintonons.(many of the units will be elites)
The best strategy is to defend yourself nonstop, by building walls and fortresses, many villagers and during defense, you will build your army. CONSUMABLES (like cows,fortresses,mines etc) help A LOT. Not to mention, that epic stuff too helps, without its nearly impossible to beat it.
First of all, build walls but leave a small hole in it, because the enemy will come through there and its easier to kill them. After building the walls, build 2-3 fortresses and as many villagers as possible to send them for gathering stuff.
Make Somathophylaxes and Hypaspists to defend, do not make archers, they will be 1 hit-kill for the enemy. Also make priests, they will be of great help with the healing/convert ability for your army.
Not too late into the mission, they will start bringing siege weapons, first rams, then ballistas and finally palintonons; if u dont have any units, send your villagers to destroy them.
If you manage to survive long enough to have resources for creating an army, the best combination i found is ballista+hippikkons. 10 b + 20-25 h will be enough to wipe one of their city.
Finally the mission requires good communication with your partner and , of course, luck.