This achievement is infact obtainable. What you need to do is knock the ball on at least 25 times in the same session as your 300th win. If the 300 wins is not your last achievement (other than "Golden Falcon" and obviously "Time to go outside now") I cannot guarantee that it will unlock for you. Both VoicelessMidget and I did the 300 wins last and unlocked the achievement through the aforementioned method.
For those who are stuck on 890GS, you can try to knock the ball on 25 times in one session, to see if it will unlock. If not, post here to show your support for a patch:
http://forums.gripshiftgame.com/showthread.php?t=40878 For everyone else, 990 is absolutely possible.
*Edit - On August 23rd (2011) a patch was released that fixed the knock-on achievement. This should mean that this achievement will now pop when you have all but this achievement. Unfortunately, it looks like people who were previously stuck on 890GS will only be able to reach 900GS.