A list of feats, items, and anything else that would help the constables mitigate the Ghost Council’s ethereal advantage, though many would work on other insubstantial threats as well. This is not an exhaustive list; it only includes things that this particular GM has dug up, and would be potentially useful for this specific party. May be updated as new stuff becomes available to select, or is brought to my attention.
Penetrating Power (controller classes only)
Ghost Scorpion Strike (works for necrotic/poison damage)
Inescapable Force (force damage only)
Ghoststrike Oil (Level 3+ Uncommon)
Ghost Strike Ki Focus (Level 2+ Common)
Tome of Undeniable Might (Level 8+ Common)
Spirit Armor (Level 14+ Uncommon chainmail)
> Compendium doesn’t mention it, but it’s an at-will power.
Pouch of Ghost-Grounding Dust (Level 15 Uncommon)
Ghostlight Candle (Level 15 Uncommon)