Play in Garden Ops mode on the easiest difficulty setting, as the Sunflower. Choose any map you like, though Garden Center works best. Advance through the first wave without using any potted plants at all, until only one zombie remains. Then, quickly plant a Scaredy Shroom plant and lure the zombie over to it when it is scared so it starts to eat it. Now begin 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 that while you step away from your console.