Powerful ultrashort pulses in the deep-ultraviolet (deep-UV) are beneficial for diverse applications from fundamental science to industrial materials processing. However, reaching high powers via conventional approaches is challenging due to three central issues: dispersion, multiphoton absorption, and optical damage. Here, we simultaneously overcome these issues with a novel fifth-harmonic generation architecture optimized for group velocity matching. We use tilted pulse fronts, including a noncollinear geometry in the final sum-frequency generation stage. This enables lower intensities and longer crystals, thereby favoring the birefringently phase matched χ ( 2 ) process over higher-order multiphoton absorption processes. Moreover, we demonstrate low-loss cascaded χ ( 2 ) -based spatiotemporal flattening of the input pulses, which enhances the uniformity of the conversion efficiency throughout the beam profile. Through these techniques, we realize a picosecond deep-UV generation source at 206 nm with record-high 2.5 W average output power and a repetition rate of 100 kHz. This result paves the way for a new era of high-power ultrafast deep-UV lasers.