Generator Rex: Agent of Providence

This is a story related achievement. It is located on the Terraria level. It comes in the room with the ramp, after exploding the 3 enemy spawns, you will go to the red screen and come back with the special move. The achievement should pop shortly after this.

This achievement is story related and shouldn't be able to be missed, but on my first play through of the campaign I did not pop this achievement. I tried to replay the mission at Agent 6, but still did not get this achievement. I had to replay the campaign up to this level and on the second play through the achievement popped.

As a side note, you may want to get any other remaining achievements you are working towards before selecting a new campaign as you will lose the progress when selecting a new game.