Nonadiabatic phase matching of high-harmonic generation (HHG) driven by few-cycle laser pulses is essential for extending harmonic energy and generating isolated attosecond pulses. However, understanding nonadiabatic HHG is challenging due to the complex interplay of various optical phases driven by temporally and spatially varying laser fields. Theoretical calculations typically rely on computationally demanding 3-dimensional simulations, which can make it difficult to extract the essential features of nonadiabatic HHG. In this work, we develop a computationally efficient 2-dimensional model that directly considers various phase contributions of HHG. Our model can well explain the experimentally observed pressure- and intensity-dependent behaviors of different harmonic orders. By appropriately parameterizing the single-atom response, our model can also estimate the variation of HHG spectra under different driving conditions. Our model can provide an efficient tool for the design and optimization of HHG-based applications.