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One of the most distinguishing properties of 2D materials is their ability to continuously tune their properties in response to applied strain, allowing strain well in excess of 10%. Straintronics developed as an emerging field enabling novel tuneable functionalities, such as band gap or effective mass tuning. Study of straintronic properties is experimentally biased by presence of substrates and in the simulations by achievable accuracy of the mainstream DFT and GW methods. Using the ultra-accurate many-body fixed-node Quantum Monte Carlo methods, we present benchmark straintronic study of monolayer phosphorene and MoS₂. We determine the atomic structures and quasiparticle band gaps in equilibrium and for any applied strain. In addition, we also determine the “phase diagrams” of crossings between the different excitations (direct to direct, direct to indirect). Our results suggest there is a colossal, larger by an order of magnitude, band gap tunability window maintaining the direct band gap in phosphorene, when compared to the quintessential straintronic material MoS₂. In addition, we ascertain that the ground state deformation energies in phosphorene exhibit negative Poisson's ratio and auxetic behavior. Comparison of our QMC results with the ubiquitous DFT methods offers insights into their biases.