How about instead of reducing the % of what they kill or how many of them die but instead bring in two types of assassins
Have one type of massing strike and one type for massing defence, you wouldnt need two types you would just need the different missions "Destroy defence" and "Destroy military"
it might not be the solution that everyone is looking for but its a start. I love using assassins there so easy to use and have a massing effect but i have to admit they are a bit op
