Every panel event, there is an enhance boost and enhance cost reduction. This way, you save lots of luna and at the same time have 150% exp rate. Save all your 1*, 2* and 3* cards. enhance your 2* and 3* cards with 1* cards.
What I usually do is save all my cards until there's an event that have an enhance bonus. I make tons of feeders and mlb some cards. I'm talking about saving 1-5k cards and try to make feeders while the enhance bonus lasts. If you have time to enhance your favorite monster, do it. It's best to level you monster from level 1 so you save even more luna. I usually just put 50-70 cards in 1 go depending on my feeder levels.