  We study Shubnikov-de Haas oscillations in a p-type WSe$_2$ monolayer under
very high magnetic field. The oscillation pattern is complex due to a large
spin and valley splitting, in the non-fully-resolved Landau level regime. Our
experimental data can be reproduced with a model in which the main parameter is
the ratio between the Zeeman energy and the cyclotron energy. The model takes
into account the Landau levels from both valleys with the same Gaussian
broadening, which allows to predict the relative amplitude of the resistance
oscillation originating from each valley. The Zeeman energy is found to be
several times larger than the cyclotron energy. It translates into a large and
increasing effective Land\'e factor as the hole density decreases, in the
continuity of the values reported in the literature at lower carrier density.
