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The study of core electron dynamics through nonlinear spectroscopy requires intense isolated attosecond extremeultraviolet or even X-ray pulses. A robust way to produce these pulses is high-harmonic generation (HHG) in agas medium. However, the energy upscaling of the process depends on a very demanding next-generation laser technologythat provides multi-terawatt (TW) laser pulses with few-optical-cycle duration and controlled electric field.Here, we revisit the HHG process driven by 16-TW sub-two-cycle laser pulses to reach high intensity in the 100-eVspectral region and beyond. We show that the combination of above barrier-suppression intensity with a long generationmedium significantly enhances the isolation of attosecond pulses compared to lower intensities and/or shortermedia and this way reduces the pulse duration as well as field-stability requirements on the laser driver. This novelregime facilitates the real-time observation of electron dynamics at the attosecond timescale in atoms, molecules, andsolids.