To what extent is the hadith narrated from Al-Hasan, from the Prophet (peace and blessings be upon him), authentic: "The severity and agony of death upon a believer is like the pain of three hundred sword strikes"?

The mursal (discontinued) hadith is a category of weak hadith. The book Tanbih al-Ghafilin by al-Samarqandi contains many fabricated and weak hadiths. One should turn away from it and towards other books that have focused on transmitting the authentic (sahih) and good (hasan) hadiths.