Use your footwork and your speed to set you up for your normal forehand, then. Against a hard hitter you just have to do this more aggressively. Try not to let his ball play you. It helps to concentrate on setting up to execute your personal "textbook" forehand every time. If the ball is coming faster just adjust the timing of your swing, ideally not the swing path. Emergency changes in the swing path are only ok if you are really out of position or something. I'm going to link
myself here to illustrate. I'm getting a variety of depths, spins, and speeds from an inconsistent opponent here, yet my swing path remains consistent. This entire video I use only one reverse finish forehand, when I am stretched wide at 1:06.