  Complex molecular structure demands customized solutions to laser cooling by
extending its general set of principles and practices. Yttrium monoxide (YO)
has unique intramolecular interactions. The Fermi-contact interaction dominates
over the spin-rotation coupling, resulting in two manifolds of closely spaced
states, with one of them possessing a negligible Land\'e g-factor. This unique
energy level structure favors dual-frequency DC magneto-optical trapping (MOT)
and gray molasses cooling (GMC). We report exceptionally robust cooling of YO
at 4 $\mu$K over a wide range of laser intensity, detunings (one and
two-photon), and magnetic field. The magnetic insensitivity enables the spatial
compression of the molecular cloud by alternating GMC and MOT under the
continuous operation of the quadrupole magnetic field. A combination of these
techniques produces a laser-cooled molecular sample with the highest phase
space density in free space.
