Expedition monster?!

I've been running some expedition content and from time to time I run into a specific monster that casts some kind of cluster bombs that deal cold damage.

I want to raise it as a spectre, but so far I've been unsuccessful in identifying the monster I need to target.

Maybe it's one of those monsters that can't become spectres.

The last map I ran, they were there. I raised 6 different kind of spectres from the location where I killed them, but none of them were it.

Anyone have any idea about what monster I'm talking about?

Druid of the broken circle?

I think I found the monster I was looking for.. Black Scythe Arbalist.

They have a sorta "Rain of Arrows" attack that forms small clusters of cold damage grenade thingies that explode after a short delay.

I'm happy I found them, but kind of disappointed that they move as slow as they do. Damage is good though, but move speed too slow for me.