Orbit-uary

If you're a dope like me and talk to Bran to start the encounter you will not get the acheive. You must talk to the golem/guy (Loremaster something-or-other) to do the encounter in hardmode, which includes the towers needed for the achieve.

Spoken to a GM regarding this achievement as you can just run in and pretty much 1 hit the boss, so I was doing that and not getting the achievement. According to the GM you have to wait for the defense systems to become active, so just sit there and let the boss hit you for a while then take him down.

Hope that helps those wanting the achievement!

To get this achievement you will first have to trigger the defenses by talking to the [npc=33686], who is located directly at the entry of the instance. Once you did so, walk up to [npc=33113] and start the fight.

The key here is to keep moving when soloing. If you think that out-leveling him with all four turrets up is a walk in the park, you will die. If you keep kiting him nicely around the room, avoiding to get cornered in, you will do just fine.