Thanks everyone who posted here, those are some fantastic ideas, and I can proudly say I just unlocked both the Silver Surfer and the Golden Age of Comics. Thanks!
I also have a few tips for anyone who might be struggling to go quickly enough on Murderworld:
Mr Fantastic: He is the easiest character on this stage. His boost, Fantastic Fists, is one of the best in the game. Max it out if possible, then pair it with Rubberband Recoil. This charge attack will kill in one hit if executed properly, allowing you to get to the bumper cars with around 9000 points, in just a few minutes. Farm for a little but. For the boss, use the boost with a move called Elastic Crush (again, maxed if possible). I took him out in about a minute.
Black Panther: The tips here will get you through, I personally just melee'd the boss. I also recommend killing everything in your path before the re-spawn point.
Storm: Seems like she would be the hardest, but really you need Lightning Strike and to keep your distance. She is devastating when paired with the Asgard Armor. And here was an interesting tip I discovered, instead of getting on the bumper cars, hang out just to the side of the entrance. There is a corner there, and the two enemies will simply run at you and you can hit them both with the Lightning Strike. Killing them both this way is far faster than the bumper cars.
Deadpool: The advice for him will also get you through, but again, I recommend trying to kill as many people on your way as you can.
One last note on the bumper cars: it is actually to your benefit to not destroy the car at the top, until you are satisfied with you score. The reason being that as long as that remains, three enemies will re-spawn instead of just two. Also, take out the Murder Bot(s) quickly, as it will blow up before too long, getting you 0 points.