Asymptotic Giant Branch (AGB) stars are low- to intermediate-mass stars in the late stages of stellar evolution. Due to their huge mass-loss and sheer number, these stars are major contributors of heavy (s-process) elements in the interstellar medium. AGB stars are ideal testbeds for understanding the mixing processes that take place in the stellar interiors. Despite its importance, the AGB is one of the least understood phases of stellar evolution, owing to the complex atmospheres and molecule-rich spectra of AGB stars. In this talk, I will present a novel method to determine the intricate atmospheric parameters of AGB stars. This method combines the recently released Gaia parallaxes and the high-resolution visible spectra with the state-of-the-art AGB models to derive the stellar parameters and abundances. With this method, we have been able to obtain observational constraints on the most crucial mixing process on the AGB namely, the third dredge-up. Furthermore, our investigation led to the discovery of low-mass AGB (initial mass ~ 1 Msun) stars. This is an evidence for third dredge-up occurrence at low-mass and solar metallicity which was not accounted for by most AGB models. Finally, I will discuss how the derived AGB s-process abundances provide crucial constraints to the galactic chemical evolution models.