The following exploit requires the 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 (precise timing required). This will deal damage to Luigi, and give you some coins. However, Luigi will stop giving coins when he reaches low health. You can use the free health block in Beanbean Castle Town (or other areas) to fully heal once Luigi stops giving coins. Repeat this as many times as desired to get unlimited coins. This trick is useful for stocking up on items.
Mario And Luigi: Superstar Saga + Bowser's Minions