I'm having about the same problems with my hitting partner who is a awful pusher. Few things you need to do:
First: Volleys are essential to beating them, get better at net.
Second, look for playing strategies that work. For example my opponent almost always crosses his backhand. I play to his backhand and know where the ball is going to, thus giving me more prep time. Learn is strategy, pushers are smart players.
Third, annoy the sh*t out of em. Pusher are players who like to be in control. I know it's not a respectful way to play, but if you want to win go all out. When you're serving take your time. When he's about to serve pick up a ball in the court and apologise. If he failed his first serve scrape your foot over the baseline (when on clay) like you're stepping in and then move back a few steps. Celebrate your shots and comment when you see he's having a bad time. Pushers tend to wear you down.