You could try a limo service. Sometimes you can get a good deal. My step-dad did this for extra income a couple of years and actually did the trip a couple of times out to MM from the beach area.
LA being LA, it can take as little as 45 minutes and as much as three hours for that drive. I have always had better luck doing the 210 route they have talked about, but it's been a long long time! I-5 is definitely the most direct route.
I hate to say it in the midst of all the Dis'ers, but I LOVE MM (Six Flags in general, actually)! Living in So. Cal., I always went there every year. I make sure you had an opportunity to check the hours for MM, though. This time of year, it starts going to very limited hours and often is only open on weekends, and only until 8pm.