There are a lot of boosts in this game, but not for the ninjas.
If a ninja dies, in its place an unit will be spawned. Ninja boosts can be achieved if the alliance will have defense ninja + 25% ninja coins (the requirements to unlock alliance chests and ninja defense will be rebalanced).
Type damage Unit spawned
Stone (petrifies) Paladin
Piercing* Archer or Arblaster
* = not available yet