  We present millimeter, optical, and soft X-ray observations of a stellar
flare with an energy squarely in the regime of typical X1 solar flares. The
flare was observed from Proxima Cen on 2019 May 6 as part of a larger
multi-wavelength flare monitoring campaign and was captured by Chandra, LCOGT,
du Pont, and ALMA. Millimeter emission appears to be a common occurrence in
small stellar flares that had gone undetected until recently, making it
difficult to interpret these events within the current multi-wavelength picture
of the flaring process. The May 6 event is the smallest stellar millimeter
flare detected to date. We compare the relationship between the soft X-ray and
millimeter emission to that observed in solar flares. The X-ray and optical
flare energies of 10$^{30.3\pm0.2}$ and 10$^{28.9\pm0.1}$ erg, respectively,
the coronal temperature of T=11.0$\pm$2.1 MK, and the emission measure of
9.5$\pm$2.2 X 10$^{49}$ cm$^{-3}$ are consistent with M-X class solar flares.
We find the soft X-ray and millimeter emission during quiescence are consistent
with the Gudel-Benz Relation, but not during the flare. The millimeter
luminosity is >100X higher than that of an equivalent X1 solar flare and lasts
only seconds instead of minutes as seen for solar flares.
