A drill you can do is to set a timer for 15 minutes and play the single rally as long as you can without missing into the timer goes off. The idea is if you struggle with using all arm on your forehand, you can create a set of conditions where your arms are completely cooked. Then you will have no option but to swing with your entire body. I also recommend doing a strength hypertrophy workout for arms, shoulders, and back. Lift until failure. With your legs and core fresh, that is another way to create the condition where you have no arms to arm the ball with.