"毫不放松的反击" can be translated as "unrelenting counterattack" or "relentless retaliation" in English, depending on the context and nuance you want to convey.
"Unrelenting counterattack" emphasizes the continuous and unyielding nature of the response, suggesting that there is no let-up or pause in the反击 (counterattack).
"Relentless retaliation" focuses more on the idea of punishing or responding harshly in return for something, with a sense of unwavering determination.
Choose the one that best fits your specific context.