Want to get most of these on the first playthrough? The x2 multiplier can be got the first time you are able to roam the city. Just head over to the far east side of the middle island, to the Power Station. Here you will play a mini-game. Smash all the other cars that spawn and the x2 brick will pop up, along with a lot of studs. Buy the brick for 100,000 (it's worth it), then play through the levels as normal (meaning smash everything and grab all the studs you can get your grubby little mitts all over). This will make it a lot easier.
Want even easier? I can do that! As soon as you get Superman and are able to use him in the open city, fly straight south to the center of the bottom big island (I add big because there's a tiny island bottom right, not that one). Here you will find the Gotham City Police department, and another whack'em'all car game. Smash the cars that spawn, same as before, buy the brick when it pops up. This is the x4 multiplier. It'll run you 200,000, but that's chump change to you what with having x2 on all this time right? Turn it on at the same time as your x2 and for the rest of the game you are getting x8 on all studs you grab! I personally had the Superhero rating on all of the missions except the first one by the time I beat the game. As soon as I finish hunting gold bricks, I'm replaying mission one and then I'll have this achievement.
If you have to replay a level to get Super Hero, neondeception mentioned in the comments that you don't have to replay the WHOLE level. After you get some of the multipliers, start up the level you need to do again, play through with your multipliers on, and when you get Super Hero, press start and "Save and finish level." He says it popped for him right there.
Nice!