DD Spectre Info (Patch 3.18.0)

Updated for: 3.18.0 (Start of Sentinel League)
Important changes from 3.17 patch notes:

Raise Spectre's most recent monster types are now saved to your character and may be Desecrated. The size of this list is capped by your Spectre limit.
Desecrate and Corpse Walk now have a static 15% chance to spawn a Spectre Corpse, rather than a 2x relative weighting compared to all other possible corpse types.
Lower the life of Spectre versions of monsters that have very high life totals. Their life is being brought down to match Kitava's Herald monsters. This will affect their corpse when created with Desecrate.

TL;DR:
Get an Auric Colossus or Auric Champion from Act 3 Solaris Temple Level 2. They are the giant golden statues. (They have 576% Life multiplier instead of the intended 360% cap. This will likely get fixed in 3.19)
You can throw away your Spectre gem after you summon one.

Explanation:
All Spectres should be capped at 360% Life, meaning you can get any monster that is 360% Life or above. (Same as Kitava's Herald)
However, Auric Champion/Colossus still has 576% Life so you should always use this.
You only need one, desecrate has a static chance to spawn Spectre corpses.
The monster type will get saved to your character, your desecrate pool will not lose it.
You may choose to let your Spectre die, or keep it alive to tank or provide some utility spell.
Usually it's better to just let it die. (+1 gem slot since you no longer need Spectre gem)

If the Auric bug didn't exist:
Arena Master has a speed buff for utility.
You can get them from Drox influenced maps.
https://poedb.tw/us/Arena_Master
Or just get a Kitava's Herald. Act 5 Cathedral Rooftop / Act 10 Rooftop/Canals/Feeding Trough
https://poedb.tw/us/Kitavas_Herald

You can look up monsters here:
https://poedb.tw/us/Monster#Monster
Sort by life, look at 360% and above.