Anybody know all the methods to raising luck levels?

The basic method which everyone knows is to use dupes of the unit, whose luck you want to raise, in levelups. However, on a few of my units, it'll just go up for seemingly no reason at all when I'm just using Tanukis or Foxes.

If there's another, or easier method to luck farming, I want in on it . Those foxes just get used up so fast and I can't keep up with the demand.