For groups of ghosts, try to stun multiple with a single Strobulb flash. When vacuuming, aim to slam ghosts into each other for chain damage. Use Gooigi to distract stronger ghosts while Luigi charges his Strobulb or positions for a slam. Remember that some ghosts have specific weaknesses; for example, some require their shields to be pulled off with the Suction Shot before they can be stunned. Practice dodging and using your burst move (ZL+ZR) for quick escapes.
