The zone is split into elemental wings, so you'll have to visit all areas of the zone to complete the achivement.
1. So, all the named wyverns and drakes seem to spawn from any wyvern or drake within the correct wing.
2. Heart of the Avalanche seems to appear in the Earth zone to the North.
3. Rockcrush seems to appear in the Earth zone to the North as well.
4. The squires each have their own static position in each of the wings.
5. I'm assuming (can't confirm) that the named captive, contained and trapped elementals spawn from the generic elemental mobs and they are dotted about. Will update further if I see any pattern.
Vengeful Captive Elemental popped in fire wing, first main square room - loc 336, -646 - he was one of the guards by the tunnel entrance.
Edited, Nov 8th 2022 6:19am by Bilderov