"完全无力回击" can be translated into English as "completely powerless to retaliate" or "utterly defenseless against counterattacks".
Here are some example sentences for context:
They were completely powerless to retaliate after the devastating first strike. （在第一次毁灭性打击之后，他们完全无力回击。）
The team found itself utterly defenseless against the opponent's relentless counterattacks. （该队发现自己完全无力回击对手无情的反击。）