Galaxies were surprisingly dusty in the early Universe, with more than half of the light emitted from stars being absorbed by dust within the system and re-radiated into far infrared (FIR, ~50-150micron) wavelengths. Dusty star forming galaxies (DSFGs) dominate the co-moving star formation rate density of the Universe that peaks around redshift, z~2, making it compelling to study them in rest frame FIR bands. From galaxies at z>1, the FIR line emission from abundant ions like O++, C+ and N+, are redshifted into the short sub-mm telluric windows. My thesis work is based on upgrading and deploying the 2nd generation z (Redshift) and Early Universe Spectrometer (ZEUS-2), a long-slit, echelle grating spectrometer optimized to study broad (dv = 300 km/s) spectral lines from galaxies in the 200-650 micron telluric windows using TES bolometers. These far-IR lines being extinction free and major coolants of the gas heated by (young) massive stars, are powerful probes of the physical conditions of the gas and the stellar radiation field. I present results from our observations of the [OIII] 88 micron line in galaxies at redshift, z~3-4, with ZEUS-2 at the Atacama Pathfinder Experiment (APEX) Telescope. To interpret our observations along with ancillary data from optical to radio facilities, we apply photoionization models for HII regions and Photo Dissociation Region (PDR) models and confirm that the galaxies host substantial ongoing obscured star formation. The presence of doubly ionized oxygen suggests hard radiation fields and often elevated ionization parameters that can only be accounted for by a population of massive stars formed during the ongoing starburst, that contribute a large fraction of the infrared luminosity. This work highlights the use of FIR line emission to trace the assembly of current day massive galaxies, details of their stellar populations and physical conditions of the interstellar medium undergoing rapid star-formation activity.