Originally posted by Nerevar:

there is not a single enemy thats immune to a damage type other than frost. and this only works because the staff is pure frost dmg only unlike the fire staff. so its probaly an oversight on thier end simply. no other weaponclass can be leveled without dealing dmg anymore.

It is entirely likely that I am mis-reading wiki[valheim.fandom.com] definitions that list

1. weak
2. resistant
3. very resistant and
4. immune

and it is also possible that this information is dated or just in error. And it is also possible that the developers may further modify the game to add some blunt component to the Staff of Frost[valheim.fandom.com] or change damage susceptibility of the golem[valheim.fandom.com]. And if so, we will adapt! What a great game!