  We describe the asymptotic behavior of conformal metrics related to the GJMS
operator in the null case, as the prescribed Q-curvature $f_0(x) + \lambda$
gradually changes. We show that if one of the maximum points of $f_0$ is flat
up to order $n-1$, the normalized conformal metrics in the lowest energy level
will form exactly one spherical bubble as $\lambda$ approaches zero using
higher order Bol's inequality. This generalizes the result of Struwe (JEMS,
2020) in the two-dimensional case to higher dimensions and helps rule out the
slow bubble case discussed by Ng\^o and Zhang (arXiv:1903.12054) to some
degree.
