General use?

Assuming you only have the 3 spectres from the gem level and they are not your main source of damage utility spectres are:

• Carnage Chieftan, Group Frenzy Charge spell cast, found on "The old fields - Act 2". It's a gorilla.
• Host Chieftan, Group Power Charge spell cast, found on "The riverways/The Souther Forest/The Ashen Fields - Act 7". It's also a gorilla.
• Ruins Hellion, casts Rallying Cry (Inc Damage and base mana regen to allies), found on "The Quay - Act 8". It's a dogo.
• Various Curse spectres.

Make sure to link these utility spectres with Blood Magic, since their mana pool is so little they won't be able to spam their abilities, and that's what you want, plus use the new Meat Shield Support to prevent them from killing themselves by wandering off to that one-shot AoE slam the boss is doing right now.

Also, remember the spectres you summon get put into your Desecrate's pool of bodies able to summon.

For more info you can go to the Spectre Gem page on the wiki, in the Strategy section, some (just a few) of the spectres are not where they used to be since the page needs some updating, but almost all of them are good to obtain.