That seems to be one of the places where it is the most difficult to get Rooter's head. I've been able to walk back and forth along the room and, as Rooter followed Ace and would stop in different positions, every once in a while, it'd be successful. The best positioning for me turned out to be to have Ace and Rooter close to each other, but their models not overlapping (as much as possible), with Rooter standing to the left and in front of Ace (so ↙ on the screen).