What can you do when you're facing a cannonball server and having trouble returning the ball? One option is to retreat at least three or four feet behind the baseline. The extra distance gives you more time to see the ball and react to the power serve.
Bjorn Borg always positioned himself yards behind the baseline, then moved in to hit the ball aggressively on his return of serve. He had time to move to the ball and shift his weight through the shot forcefully.
A second idea against a booming serve is a shorter backswing. A long stroke takes more time and is harder to control: two definite drawbacks against a hard serve.
Curtail your backstroke and think about contacting the ball early. Don't worry about generating sufficient power. The cannonball server is supplying that; all you have to do is meet the ball solidly.