induration. 1. the process of hardening or being hardened.
1 They're weak against the Ice and Water types in reference of erosion, which cause this elements to the ground. 2  They're immune against the Electric type because in real life, the ground is a natural sink of electrons.  The Bug type resists them because many bugs like worms live inside the ground and many of them eat it.
Steel-types completely wall this set, so it is ideal that your team has Pokemon that pack strong Fire-, Fighting- or Ground-type attacks. Moltres is an excellent partner, as it boasts STAB on its powerful Fire Blast and is immune to Skuntank's only weakness, Ground-type attacks.