"毫不放松的反击" can be translated into English as "a relentless counterattack" or "an unrelenting counteroffensive", depending on the context and the level of intensity you want to convey.
"Relentless" implies persistent and unyielding effort, suggesting that the counterattack will continue without pause or weakening.
"Unrelenting" carries a similar meaning, emphasizing a lack of mercy or cessation in the attack.
Both expressions are suitable for describing a determined and unwavering response to an attack or provocation.