The easiest thing to do is step back so you can strike the ball in your comfort zone. Then hit the same moonball shot back- deep and with some topspin (if you can). Don't get in a hurry or be embarrassed by how stupid it looks. Continue hitting deep moonballs back until, eventually, you get a shorter ball- then you can move up and drive the ball for a winner or, at least, put some pressure back on the opponent.
There are about half a dozen other things you can do- but they would require some skill- achieved through practice. So, you can start with this- then work with your practice partner on other skills- taking the ball on the rise, lobbing over a net-rusher, drop-shotting the guy who hangs 8 feet behind the basekine, striking/slicing through a high ball with power- and eventually you will have more options.