What is the remedy for a person's fear that his deeds will not be accepted on the Day of Judgment, or that he will not reach the highest ranks of Paradise, despite his striving and diligence in acts of obedience and avoiding reprehensible things? And is having a good opinion of Allah, along with taking the necessary means, sufficient for him to reach Firdaws (the highest level of Paradise)?

A Muslim is obligated to strive in obedience to Allah while maintaining a good opinion of Him. Fear that one's deeds may be rejected should be a motivation to improve them, not a deterrent. Whoever is overtaken by death in this state is upon goodness. The Muslim should ask Allah for Paradise, knowing that Allah's mercy is the cause of entering the Gardens, and that deeds are merely means and not the price of Paradise. He should ask Allah for His mercy, turn to Him with his heart, be sincere in his supplication, perform the obligatory duties, avoid prohibitions, and increase in supererogatory acts, then hope for good from his Lord.