Frame theory applied to source field decomposition facilitates the decomposition of an incident plane wave into a set of paraxial Gaussian beams. Spatial and spectral localization properties of such beams allow to bounce them through multiple reflections when reflecting surfaces are smooth and large enough as compared to wavelength. Gaussian beam spectral localization also yields dramatic reduction of the number of beams contributing to monostatic scattering cross section. An algorithm taking advantage of those properties is presented and applied to scattering by a set of blocks simulating an urban-like environment, with large dimensions as compared to wavelength. Numerical results will illustrate the method efficiency and will be compared to Fast Iterative Physical Optics results.