Headbangers: Rhythm Royale

DISCLAIMER: This information is correct as of season 1 - things may get easier or harder as seasons progress - comment if something is wrong about this later and I will amend the solution.
The guide will be the same for all tournament wins.

If you go for the all 9 challenge cards, (most of) these minigames will likely be so burned into your memory with practice that you can wipe a normal lobby without too much effort.

The safest method for quick wins is to fill your lobby with bots. You can do this by turning off cross-play and changing your region to a region that is asleep (or an asian region any time of day). Bots will be deceptively difficult in final rounds (particularly After Party), but likely less problematic than human players.

If you want to win real matches, the thing I can confirm from my experience is that you don't have to be amazing at stages 1-3, only passable. There are some techniques to do well in those, which you can find in the guides for the challenge card #1-#9 achievements. The main thing you have to get really good at are the final rounds. Here are a few tips for them:

1. WEAR HEADPHONES. After Party is much easier when you can hear the beats in the background, which you can use to time button presses without the sliding line visible. It also helps remove any external distractions.
2. This may be a controversial opinion, but grab no item boxes in the final round, ever. Almost every time I play final rounds with a person grabbing the item boxes, they'll grab it and either get distracted and miss a note, or they'll get the item box which backfires and makes them lose instantly. If you stay focused on the game and don't think about the boxes, I did much, much better.
3. Make sure to do anything possible to reduce latency - don't have streaming services in the background, use a wired connection if possible, and make sure you recalibrate in the settings to make sure your timings are perfect.