  We perform a series of dimensional reductions of the 6d, $\mathcal{N}=(2,0)$
SCFT on $S^2\times\Sigma\times I\times S^1$ down to 2d on $\Sigma$. The
reductions are performed in three steps: (i) a reduction on $S^1$ (accompanied
by a topological twist along $\Sigma$) leading to a supersymmetric Yang-Mills
theory on $S^2\times\Sigma\times I$, (ii) a further reduction on $S^2$
resulting in a complex Chern--Simons theory defined on $\Sigma\times I$, with
the real part of the complex Chern-Simons level being zero, and the imaginary
part being proportional to the ratio of the radii of $S^2$ and $S^1$, and (iii)
a final reduction to the boundary modes of complex Chern--Simons theory with
the Nahm pole boundary condition at both ends of the interval $I$, which gives
rise to a complex Toda CFT on the Riemann surface $\Sigma$. As the reduction of
the 6d theory on $\Sigma$ would give rise to an $\mathcal{N}=2$ supersymmetric
theory on $S^2\times I\times S^1$, our results imply a 4d-2d duality between
four-dimensional $\mathcal{N}=2$ supersymmetric theory with boundary and
two-dimensional complex Toda theory.
