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 crumb amounts.

If you are going for all nine challenge cards, you will likely get more than 50k crumbs naturally - inversely, if you aren't going for all achievements, you will have enough time to at least complete some of them while grinding for crumbs. Here are a few tips, if you do not get this one naturally:

1. Having a turbo function on your controller would likely help with the bonus round where you have to "stretch" (press A repeatedly). A very minor crumbs gain, but it could end up getting you a few extra hundred crumbs every time that comes up (I personally cannot beat a ~540 score manually).
2. Although I would recommend playing with people, changing your preferred region in the settings to a region which is asleep (or the Asia regions, any time of day) will likely put you in a lobby full of bots. Bots are useful for crumbs, as they are easier to defeat to hit later stages, and they also always spin for bonus rounds, whereas players typically do not during hard sequences. You can end up with two bonus rounds per game if lucky, doing it that way.
3. If you get to level 39, you will have earned 67k crumbs for completing XP levels - it goes without saying that if you haven't got a high XP level, you should go for it (level 36 and 39 in particular, which give 20k crumbs each).