1) Send 500 calvary or 1000 pikes to garrison the city you want to take. (Using pikes the abatis do not have to be demolished)
2) Once they are there, one player gets the boot/invite from alliance.
3) Attack the city with one scout
When your hero arrives with the scout....he commands your garrisoned troops already there and they fight together to take the city.
So the garrison gets there 5 or 6 times faster than you would if attacking and the hero/scout gets there quick
You will have the city so fast it might make you do this