High-harmonic generation (HHG) normally requires a careful adjustment of the driving laser intensity (typically 10 14 − 10 15 W/cm 2 ) and gas medium parameters to enable good phase matching conditions. In contrast with conventional wisdom, we present experimental results indicating phasematched HHG in all rare gases, using a high-density medium and a driver laser intensity of around 10 16 W/cm 2 . The experimental results are corroborated by theoretical simulations, which indicate that ionization-induced self-phase modulation and plasma defocusing self-regulate the driver laser intensity to a level that is appropriate for good phase matching. A ten-fold broadening of the NIR spectrum is observed, which results in the generation of continuous spectra from 18 − 140 eV in spite of using 50-fs-long driving pulses. The presented scheme represents a simple and versatile concept for the generation of XUV and soft X-ray continua, which are ideally suited for transient absorption and reflection spectroscopy.