Consider the function $f(x, y, z)=x^{4}+2 y^{3}+z^{2}$. The directional derivative of the function at the point $P(-1,1,-1)$ along $(\hat{\boldsymbol{\imath}}+\hat{\boldsymbol{\jmath}})$, where $\hat{\boldsymbol{\imath}}$ and $\hat{\boldsymbol{\jmath}}$ are unit vectors in the $x$ and $y$ directions, respectively, rounded off to $2$ decimal places, is $\_\_\_\_\_\_\_\_$