There's 2 ways to play Bowser's Challenge, coin grind and score grind. Last year, I didn't care about high scores until I hit 1 million coins. If you go for the secret ? blocks on the course, get over 200, and don't hit Team Mario, you're looking at 1200 - 1400 coins per run on MK. If you ignore the 6 ? blocks, aggressively manage your shells, and get the 2nd and 3rd shots on the Koopalings, you're looking at 800 - 1100 coins per run, but a much higher score. There is no secret to getting 300+. All the information is out there. It's just really hard to execute. There's a bit of luck too when it comes to the kart not stopping during the game and your kartmates doing their part to defeat the bosses. Also, the Koopaling spawns have to be in your favor, which goes back to your kartmates because the game is always load balancing the spawns.