This achievement is for killing the next boss after the Overseer. This achievement is story based and hard to miss.
Soon after you defeat the Overseer you will be taken to a rather large Estherian Ruin Antechamber by Syl.
She will tell you that to reach the Root Golem you must activate a main portal. Well the Portal will need 3 keys, meaning there are 3 other separate secondary portals, all containing 1 key each.
Each portal has a 2 floor dungeon, with a few champion npc's, and nice loot. It is recommended to clear out these dungeons all the way through to boost your fame a large chunk.
After you obtain the 3 keys (Called Glyphs in game) you will head back to the Antechamber and talk to Syl. She will tell you that you must go through the main portal alone.
When you enter the portal there will instantly be a large group of enemies surrounding your left and right side. These are mainly large groups of easy enemies, but be careful just incase.
If you clear out the outer enemies this boss will be no trouble at all. In fact the Overseer proved more of a challenge to me than the Root Golem.
The Root Golem despite what the achievement says, looks nothing like a golem. The boss's name is Corrupted Ancient and is a tall red and silver looking beast. As soon as you kill it this achievement will pop. Boom there ya go, good job guys!