(Requires Hammer duo action [Mini]) Shrink Mario with a hammer, then switch to High Jump. Activate the High Jump, and have Mini-Mario jump while Luigi's above him. (The timing's a bit strict.) This will deal damage to Luigi, and give you some coins. However, if Luigi goes to low health, he'll stop giving out coins. This is easily infinite, as you can just use the free health block in Beanbean Castle Town (or other areas) to heal up fully once Luigi stops giving coins. Could be useful if you just want to stock up on a bunch of items.
