As a necromancer fighting unit, the skeleton is easy to tear apart, so their tactic is to ambush or overwhelm with numbers and hit you with weapons before you can disable them. So they're best against unarmored opponents.

Archers might be more dangerous if they are strong enough to pull a war bow like a longbow.

Clever necromancers will make bone molds and craft critical bones out of ceramic or bronze or steel in order to make more resiliant units, but these often require different animation spells. Some expeimentation has been done with bone powder and ceramic.