In this study, we proposed a scheme for generating high-frequency terahertz waves in MgO∶LiNbO₃ slab waveguides using optimised cascaded difference frequency generation at polariton resonance. Unlike previous methods for generating high-frequency terahertz waves with difference frequency generation by two near-infrared lasers, we employ our proposed scheme to generate high-frequency terahertz waves by the simultaneous interaction of a series of cascaded optical waves. With hundreds of cascaded Stokes processes, where the phase mismatches are optimised by adjusting the dimensions of the slab waveguide, high-frequency terahertz waves are efficiently generated. Moreover, terahertz wave intensities are further enhanced with the gigantic nonlinear optical coefficients at polariton resonance of MgO∶LiNbO₃ crystal and the reduced terahertz wave absorption coefficients by the slab waveguide. The energy conversion efficiency from pump wave to terahertz wave exceeds 40% using this method at room temperature. This study presents a novel scheme for generating high-power high-frequency terahertz waves, which can promote the applications of high-frequency terahertz waves.