We present ultrawide flat-band reflectors enabled with multilevel resonant leaky-mode structures. The reflectors are designed using particle swarm optimization. We show that three-level silicon-on-insulator structures provide bandwidths of $\sim 840\mathrm{nm}$ for TE polarization and $\sim 835\mathrm{nm}$ for TM polarization, across which reflectance is greater than 99% and 97.5%, respectively. For the germanium-on-insulator system, the 99% TE and TM bandwidths are $\sim 1100\mathrm{nm}$ and $\sim 1050\mathrm{nm}$ . The results indicate the potential of multilevel resonant leaky-mode elements in electromagnetics and photonics.