  For a graph invariant $\pi$, the Contraction($\pi$) problem consists in,
given a graph $G$ and two positive integers $k,d$, deciding whether one can
contract at most $k$ edges of $G$ to obtain a graph in which $\pi$ has dropped
by at least $d$. Galby et al. [ISAAC 2019, MFCS 2019] recently studied the case
where $\pi$ is the size of a minimum dominating set. We focus on graph
invariants defined as the minimum size of a vertex set that hits all the
occurrences of graphs in a collection ${\cal H}$ according to a fixed
containment relation. We prove co-NP-hardness results under some assumptions on
the graphs in ${\cal H}$, which in particular imply that Contraction($\pi$) is
co-NP-hard even for fixed $k=d=1$ when $\pi$ is the size of a minimum feedback
vertex set or an odd cycle transversal. In sharp contrast, we show that when
$\pi$ is the size of a minimum vertex cover, the problem is in XP parameterized
by $d$.
