Is it incumbent upon the believer to love death and not fear it, and is there an act that guarantees Allah's mercy and Paradise?

A believer is not obligated to love death, and their natural aversion to it is not blameworthy in Islamic law, nor are they tasked with eliminating it, as it is beyond their capacity. However, the aversion to death is blameworthy in Islamic law for those with weak faith who are heedless of the hereafter. As for guaranteeing entry into Paradise, it is through Allah's grace and His granting of success to His servant through faith, obedience, and adherence to the Sunnah. So, whoever fears Allah, obeys His commands, avoids His prohibitions, and remains steadfast in His obedience, Allah will have mercy on them and admit them into Paradise.