For The King

Look for a scourge hex area called “warlocks volcano” and the achievement will pop as soon as you defeat Coal Heart (mage with high magic armor but no physical armor). Mine was level 7 and had two lower level features with him on the “for the king” campaign map in “the parched waste” area. The game is randomized so this might not always appear. This also appeared for me in the “Frost Adventure” campaign, and was level 4 (I found it early on) so seems like the game scales these depending on about what level you should be when you encounter them.