Elusive 4 achievement / where can we find them / post pictures

[Taejo]

The bosses seems to have set spawn locations though there typically seem to be multiple in each map.

Yep, they will be spawned in place of a mob of the same likeness:

Skratch = skeleton warriors
Ripper = sharks
Amra-zu = giant demon (ones with hook hands)
Argex = giant ice elementals

Anywhere on a map where those types of mobs spawn is their set location. And as Haligali discovered, the skeletons that are summoned from Necromancers can also be Skratch - so let them spawn their 8 skeletons for maximum chances at Skratch!