\[- \left( - x^{2}y^{4} \right)^{4} \cdot \left( 6x^{4}y \right)^{2} =\]
\[= - \left( ( - 1)x^{8}y^{16} \right) \cdot 6^{2}x^{8}y^{2} =\]
\[= - x^{8}y^{16} \cdot 36x^{8}y^{2} =\]
\[= - 36x^{8 + 8}y^{16 + 2} = - 36x^{16}y^{18}.\]