Recent experimental work  suggests that the increased motility of cancer cells observed in a confluent monolayer of normal cells is due to the mechanical mismatch between the two cell types. The soft cancer cell undergoes large deformations and can squeeze between small channels defined by the space between the normal cells. We developed a phase-field model description of cellular monolayers to study such a process. The system is setup as a free-boundary problem where each cell is a highly deformable soft body . The motion of each cell is described by a persistent random walk where the velocities and persistent times can be obtained from experimental observations. Our model is used to test the hypothesis that the mechanical mismatch alone increases the motility of softer cells when all other parameters are the same. Lee et al. Biophys. J. vol. 102 p. 2731 (2012). Najem et al. Europhys. Lett. vol. 102 p. 16001 (2013).