The high-beta capability of the spherical tokamak, coupled with a suite of world-leading diagnostics on MAST, has facilitated significant improvements in the understanding of performance-limiting core instabilities in high performance plasmas. For instance, the newly installed Motional Stark Effect (MSE) diagnostic, with radial resolution < 25mm, has enabled detailed study of saturated long-lived modes in hybrid scenarios. Similarly, the upgraded Thomson Scattering (TS) system, with radial resolution < 10mm and the possibility of temporal resolution of 1μs, has allowed detailed analysis of the density and temperature profiles during transient activity in the plasma, such as at a sawtooth crash. High resolution Charge Exchange Recombination Spectroscopy (CXRS) provided measurement of rotation braking induced by both applied magnetic fields and by magnetohydrodynamic (MHD) instabilities, allowing tests of neoclassical toroidal viscosity theory predictions. Finally, MAST is also equipped with internal and external coils that allow non-axisymmetric fields to be applied for active MHD spectroscopy of instabilities near the no-wall beta limit. Such resonant field amplification measurements suggest that MAST has been able to operate above the no-wall limit. In order to access such high pressures, the resistive wall mode must be damped, and so numerical modelling has focussed on assessing the kinetic damping of the mode and its nonlinear interaction with other instabilities. The enhanced understanding of the physical mechanisms driving deleterious MHD activity given by these leading-edge capabilities has provided guidance to optimise operating scenarios for improved plasma performance.