This work presents a perturbative approach for the evaluation of the effective oscillator strength averaged over continuum and Gaunt factors, for a particular case of complex atoms with two electrons outside closed shell. Based on the Coulomb formalism accounting for the general properties of the Coulomb Green's function, together with its available compact integral representations and discrete state expansions, results refer to Be-like C, Al and Ar, and Ba-like W ions, as an example. Such calculations are of interest as they represent accurate benchmark data for beam emission spectroscopy, Zeff analysis or complex atoms modeling in fusion plasma devices.