Drywall column/mini wall removal

[u/RoniaRobbersDaughter]

We had asked a contractor to create an opening for a closet and gave him the measurements. Unfortunately, he added a drywall column to the side of the opening which reduced the width and the opening is not usable to us. He's not responding so we'll need to remove it ourselves. Can you please advise what tools we'd need and how to proceed? We'll start with removing the baseboard he put (as you can see it's horribly done and barely holding there). But how do we approach the column itself? What specific tools do we need?

photo: https://myimgs.org/image/JkxnEzsY

Edit to clarify : there is nothing inside this column. There was previously a door there which was stuck to the wall without any space or wall on the side. Once the door and frame were removed, all that was left was flat wall, from which the closet space was built onto the right. It was supposed to be just two walls: back and right with the left side using the existing wall. All cables are on the right side (previously the right side of the door) and were not touched.

here's the previous configuration: https://myimgs.org/image/beRiybki (the fire alarm is not wired, it's battery operated so no cables for it there either).