I tossed a smaller achilles in a tank with a pbt and chocolate who had been there for about a year. Within a couple of days, the achilles was almost dead. (He was perfectly healthy in QT for 6 weeks and when introduced.) I tried many things none worked. Eventually I tried mirrors. I bought 2 1x4 mirrors and taped them to the glass along the front side. EVERYONE in the tank attacked their reflections nonstop. The less aggressive fish gave up first, and then after a week, I went down to one mirror. After another week, I cut the remaining mirror in half and taped it to the end of the tank. They would still attack their reflections, but not each other. Since it wasn't a viewing side, I left it up for months. Now there is no mirror and the two tangs hang out together most of the time.
Try the mirror trick. It's awesome. I was worried at first they would hurt themselves bashing against the glass, but no problems.