Play Garden Ops mode as the Sunflower on the easiest difficulty setting and select any map, although Garden Center works the best. Proceed through the first wave without using any potted plants at all, until only one zombie remains. Once this occurs, quickly plant a Scaredy Shroom plant and lure the zombie over to it when it is scared so it starts to eat it. Now start healing the Scaredy Shroom using the LB ability, which will quickly generate coins to the tune of about 20,000 per hour, as long as you keep it going. You can even do this while you step away from your console.
While playing as the Cactus, stand on the edge of a tallnut barricade or wall and press the button that activates your drone. Fly beneath the vantage point where your Cactus is positioned, and from there ascend to position the Cactus on the flying drone you are controlling. You can fly up to a roof or other high elevation, reaching areas that are normally unavailable to that class. This can also be done with the Engineer's drone but it doesn't work as effectively.