Thermal interface materials (TIMs) with high thermal conductivity, fluidic characteristics, and surface wettability are crucial for the effective thermal management of high-power electronics. Gallium-based liquid metal (LM) can be a candidate for obtaining the purposes due to their thermo-physical properties. Several studies attempted to further increase the thermal conductivity of composites by adding conductive fillers. However, these efforts have been limited due to the oxidation and solidification issues at high vol% of fillers. In this work, we apply three strategies to obtain the enhanced thermal conductivity while maintaining the fluidity: 1) suppressing Ga-induced oxidation, 2) reducing the matrix-fillers interfacial resistance, and 3) forming additional heat-pathways within the LM matrix. An oxide-free ultrasonication-assisted particle internalization method has been developed, in which the copper nanoparticles (Cu NPs) are incorporated into the gallium-indium-tin (GaInSn) LM matrix. The fabricated composite shows similar to 180% enhancement of thermal conductivity (similar to 64.8 Wm(-1) K-1) with only 4 vol% of nano-fillers compared to the untreated GaInSn. In the droplet impacting test, the synthesized GaInSn/Cu NPs composite presents almost identical spreading diameters with the untreated one, which confirms the maintained fluidity. The predicted thermal conductivity using the theoretical model, calculating the measured cluster fraction, is well-matched to the experimental data, which clarifies that the nanoparticle clusters created effective heat-pathways. A high-quality interface with a silicon substrate is confirmed by the X-ray photoelectron spectroscopy, demonstrating the presence of a thin-film Ga2O3 adhesion layer. In the cooling performance test, the developed TIM provides over 20% lower hot-spot temperature than that of the grease-type TIM at high heat flux regime (>150 W/cm(2)), showing excellent thermal stability over 230 cycles of acceleration test. This work will help develop high-performance TIMs, especially for high power density semiconductor applications.