The non-linear matter power spectrum will be one of the most important tools to discriminate between competing cosmological models in the upcoming years. However, apart from some N-body simulations in specific models, there are few predictions of the non-linear power spectrum for models beyond LCDM (such as scalar-tensor theories) that have the required few-percent accuracy to be tested with future surveys at least in the mildly non-linear regime. In this talk I will review the standard perturbation theory for the Vlasov-Poisson system of equations in the field theory language, give a glimpse of how Renormalized Perturbation Theory (RPT) and the Time Flow formalism work and then will explain how we plan to modify it in such a way that it can handle modified gravity terms, such as the ones obtained from Horndeski's theory in the quasistatic limit.