I'm so dumb. I had no clue how the multiplier worked, so the 5k point medal seemed hopeless. Once I finally realized that items I send into the crowd are tossed back, it became somewhat easy. I'd just focus on one item near the back of the stage, and try to avoid the rest. Once the 100x multiplier is reached, rinse and repeat. Getting 2 to 3 100x multipliers is enough to exceed 5k points. It's also fun to let yourself fall off the cliff, then mash the buttons on the left. You'll fly into the air and fall through the roof of the concert hall.
Anyway, it's a simple game, but I had a ton of fun playing it.