question

since berserk converts phys into magic damage, does adding a guardians light -magic res + magic skill dmg on the arreats help? or only helps on hammerdin?

Berserk converts 100% physical weapon damage to magic damage. Conversion occurs after +% Damage and a critical hit from a weapon Mastery, Critical Strike or Deadly Strike are applied and before resistance is applied.

The +magic skill dmg probably won't help, but the -magic resist probably will.

the -magic should apply i guess

On the plus side, that's the good part anyway.

as a rule, +% skill damage effects don't apply to damage converted to that type. Cold arrow's % physical to cold, same with fire, magic arrow, berserk
-% enemy resist still works of course