Attacking with scouts allows you to bypass all wall defenses and damage the highest combined attack units in the city.
100k archers are equivalent to 35k ballista or 25k catapults.
To calculate the number of ballista you can safely keep in a city, multiply your number of archers by .35 ( .25 for catapults ) then subtract 1.
Example: if you have 160k archers in your city, you need to have fewer than 56k ballista ( .35 x 160,000 ) to keep them safe from scout bombs.