We propose and experimentally demonstrate a generic framework, requiring neither active components nor complex designs, for undistorted asymmetric elastic-wave transmission in a thin plate using just a layer of lossless metasurface. The asymmetric transmission stems from the uneven diffraction of the +1st and −1st orders on opposite sides of the metasurface. Compared with previous loss-induced strategies, the present metasurface provides nearly total transmission on the transportation side, but total reflection on the opposite side, exhibiting a higher contrast ratio of the transmission. Moreover, we show that this strongly asymmetric behavior is robust with respect to the incidence angle, the frequency, and the effects of loss. The present work paves the way to compact rectification, high-resolution ultrasonography, and vibration and noise control in elastodynamics and acoustics.