First, see those emblems on the map? The crossed swords? Those are the locations of the Guard Brutals. But, if you already knew that and wanted to know where the Wild Brutals are, I've got stuff for that too.

Rat King: swamp south of Vamp'ire.
Tusk Trio: Borewood
(Whatshisname) The Invisible: Kamengrad Ruins
Mother of Wolves: forest south of Avan
Styx's Servant: Angel Falls
Lernaean: Dreary Mire
Enigma: find the Riddlers, get a totem from the Riddlemaster, and the Enigma will follow you almost everywhere (besides villages, where you're safe)
Andesia's Bane: in the Ruins, after you free it (light fire south-east of the Ruins; follow maroon trees)