For high-energy-density supercapacitors, two-dimensional (2D) MXenes are being increasingly explored due to their inherent conductivity and excellent chemical properties. However, MXenes failed to achieve high power density and exceptional stability. Addressing this, we report the fabrication of an asymmetric supercapacitor with nickel MXene (cathode) and nitrogen (N), sulfur (S), and phosphorus (P) self-doped biomass-derived activated carbon (anode). Detailed structural and chemical characterization studies reveal layered nanosheets in NiMX caused due to solvothermal etching cum exfoliation and unique micro-mesopore distribution in the optimized Euphorbia milii plant leaf-derived heteroatom self-doped activated carbon (EMAC-700) because of KOH activation. NiMX and EMAC-700 delivered high capacitances of 474.3 and 575.8 F/g, respectively, at 1 A/g with a 6 M KOH electrolyte. This is attributed to the pseudonature of NiMX and the presence of heteroatoms and the large surface area (2349 m2/g) of EMAC-700, facilitating fast electrolytic ion transfer. Finally, an asymmetric device with NiMX//EMAC configuration in 6 M KOH delivered a 152.6 F/g cell capacitance at 0.5 A/g under 0-1.5 V. Additionally, an ultrahigh energy density of 47.6 W h/kg at a 375 W/kg power density was achieved along with an 81.7% capacitance retention after 30,000 cycles at 15 A/g, signifying its potential for next-generation energy storage applications. © 2023 American Chemical Society.