Most of the customisation items for characters are locked away in the Krypt mode, which requires you to spend coins to open boxes that contain random items. The boxes with the better items are much more expensive than the others and drain resources quickly.

What's the quickest/most efficient way to grind for the coins required to open a bunch of chests for the best chance at getting the items you want?

Note: The in-game store lets you purchase specific items, but it's only about 5 at a time and they're on a rotation that takes many real-world hours to rotate. It's not a viable solution to the problem of getting the items you actually want.