Interaction between energetic electrons and plasma waves resides in many fundamental plasma processes. A recent study using single particle analysis on the mechanism of particle-wave interaction and pitch angle scattering of the particles [1,2] revealed that electron motion in an optimal coherent R-wave or whistler wave is sensitive to the initial condition, and an instantaneous pitch angle scattering is expected. Here, we reproduce this result by self-consistent two-dimensional particle-in-cell simulations. Then, we explore the so-called firewall effect on the electron acceleration of the R-wave. We verify that the growth of bump-on-tail in the electron distribution can be inhibited by external whistler waves. We discuss (1) the required power balancing against collisional damping and (2) the timescale of pitch angle scattering compared to the prediction from the quasilinear diffusion theory. We propose the injection of plasma waves as a promising method to mitigate energetic electrons called runaway electrons (REs) that can threaten machine safety [3]. This work was supported by NRF of Korea under grant no. RS-2022-00154676, RS-2023-00281272, RS-2024-00409564. The computational resources for the simulations are provided by the KAIROS supercomputing system at the Korea Institute of Fusion Energy (KFE). References [1] P. M. Bellan, Phys. Plasmas 20, 042117 (2013) [2] Y. D. Yoon et al., Phys. Plasmas 28, 060702 (2021) [3] Z. Guo et al., Phys. Plasmas 25, 032504 (2018)