*GOD Rings/Overlord's Ring: Buy these as you need to so that you can evolve your Cards to GOD/DRA. The Medal Shop is the only consistent way to get these Rings outside of RNG drops with Training. Make sure you always have enough Dragon Medals to exchange for these!
*Deck Cost +10: These add much needed extra Deck Cost to your Deck. They're pretty cheap for the most part.
*Skill Guarantee W-Link: This SP Skill will make your life much easier once you start moving up towards the higher difficulties, allowing you to Link two cards at once. It's somewhat expensive, but it's worth it in the long run.
*Yata no Kagami: A incredibly useful Sub Card once you start moving up towards the higher difficulties. Being able to have useful Support cards activate their effects twice comes in handy and it blocks linking with Enhance cards if it activates.