Scintillators that sustain efficient radioluminescence at high temperatures are essential for next-generation radiation imaging but remain challenge, particularly for hybrid halide systems. Herein, we report an organic-inorganic hybrid scintillator, ETPP2MnCl4, in which spatially isolated [MnCl4]2- tetrahedra are embedded within a zero-dimensional host-guest framework formed by bulky ETPP+ cations. This architecture suppresses nonradiative relaxation and energy migration, yielding intense green emission from the Mn2+ 4T1(G)→6A1(S) transition with a photoluminescence quantum yield of 45.2%. Remarkably, ETPP2MnCl4 exhibits a thermally activated enhancement of radioluminescence, which is attributed to reverse intersystem crossing (RISC) from the triplet to singlet states of the organic ligand and then efficient energy transfer to Mn2+ activators. As a result, it exhibits anti-thermal-quenching behavior, retaining 112% of its radioluminescence at 130 °C, and maintains over 92% of its emission intensity after prolonged high-temperature and high-dose irradiation. Under X-ray excitation, the material delivers a light yield of 20040 photons MeV-1, a detection limit of 49.8 nGyair s-1. and a spatial resolution of 12.2 lp·mm-1. These findings demonstrate a previously unreported RISC-assisted sensitization mechanism in hybrid scintillators and establish a design strategy that integrates triplet-singlet conversion pathways with discrete metal halide emitters, positioning ETPP2MnCl4 as a promising scintillator for extreme-environmental applications.