To obtain this achievement, it requires you to complete a Challenge Room. Challenge rooms show up on the realm map as a point of interest icon just like this-> [ ! ]. You'll know you've found one because the room will contain a pedestal, on top of which sits a small pyramid shape with a black space cut out of the middle.
Going up to it and pressing the interact button(X by default) will activate the challenge, which consists of several waves of increasingly difficult enemies. Successfully kill them all to complete it and earn the achievement.
My first challenge win was done in the first realm(level), but I did not get the achievement. I later completed another challenge in the second realm and it popped right away. Not sure if it was a bug or if early game ones don't ount, but something to be aware of going forward.