The generation of picosecond shear strain pulses for ultrafast magneto-acoustic and acousto-optic applications remains challenging. We utilize the effect of acoustic mode conversion at the interface of acoustically isotropic and anisotropic media in order to generate picosecond shear strain pulses with high amplitude. We use a metal opto-acoustic transducer deposited on low-symmetry (311)-GaAs commercial substrate and excited by femtosecond laser pulses. We monitor propagation of the strain pulses in the substrate by means of the picosecond acoustic polarimetry, which is based on the rotation of the polarization plane of the probing laser beam due to the strain-induced optical anisotropy. By probing at the side opposite to the transducer, we are able to separate in time and then analyze the strain pulses of different polarizations. In the following experiments, we show that the picosecond shear strain pulses generated by this technique can induce significant changes in magnetic anisotropy of thin ferromagnetic films resulting in ultrafast modulation of magnetization with modulation depth up to 10%.