Is there an adjective for "cannot be defended against"?
Solution 1:
Unstoppable (Dictionary.com)
(adjective) that cannot be stopped or surpassed; unbeatable
Uncontrollable (Dictionary.com)
(adjective) incapable of being controlled or restrained
Solution 2:
Why not irresistible
not able to be resisted or refused; overpowering [Collins]