Quoted from Rcade:

This is by design, and is not sloppy implementation. And while I DO think having Xavier as the SSS is a little crazy, it IS a SSS and should be hard to get.
You CAN get it from the upper flipper off the soft plunge, but that's pretty much a luck game, and most of the time, you're better off just trying for the Psylock SS and hoping it bumps off the post into Xavier. So there's no skill there.
The TRUE skill to this shot, and why I like it, is in the fact that you soft plunge and let the ball drop onto the right flipper and make Xavier. But it's not always as simple as that. The soft plunge drop, more often than not, will drop onto the right sling, causing the ball to rocket around the lower playfield, which is why the HFC and BH targets and slingshots don't kill the SSS. When this happens, it's a crapshoot as to whether or not you can get it controlled in time (let alone on the correct flipper) to make the Xavier shot (which is a TOUGH shot to make on command) and get the SSS in time. The KEY to making it as CONSISTENTLY as possible is to:
-soft plunge, but don't hit the Pheonix switch
-as the ball rolls down and reaches the end of the upper flipper bat, SLAP THE RIGHT SIDE OF THE CABINET NEXT TO THE UPPER FLIPPER BAT!! This bumps the balls trajectory slightly to the left, and makes it MUCH more likely that the ball will miss the slings. This same "move" is used a lot in games like ACDC to keep the ball out of the slings during an orbit.
-ball will miss the sling, then cradle with the right flipper bat, aim and make the Xavier shot.
One thing I will give XM, this is a TRUE SSS. Yes, you can make it a luckfest, but it takes a good player to make it by design.