The meaning of the supplication ‘wa lā yanfa`u dhal-jaddi‘ (the might of the mighty person cannot benefit him against You) is that the rich person will not benefit from his wealth on the day of judgement, Allah SWA says: (The Day when neither wealth nor sons shall benefit, except him who comes before Allah with a pure heart) rather what benefits him are what good deeds he has done, which we hope Allah accepts.

Sheikh Al-Mubarakpouri says when explaining the book of Sunan Al Tirmidhi (The mightiest of the mighty cannot benefit) means the rich person will not benefit from his wealth, rather he will benefit from his good deeds. 

Al Hafidh ibn Hajr says in Fath Al Bari, the explanation of Sahih Al Bukhari: “Al Khatabi says: The mighty- is the rich person, or fate, but An Nawawi said: the most correct opinion is that it is his fate in this world that makes him have lots of money, or children, or power, or kingship, and the meaning behind it is: His fate will not save him from Allah SWA, rather what will save him is Your mercy and Your Grace…”