Just wanted to share a tip, please add your tips & tricks as well!!
I have heard through several people that you should change your attack cards during each fight. So I prepare 3 combo spells before a fight, and match a mystic/fairy spell with each combo (so 3 cards per attack).
This powered up all of my mystic and fairy cards, which tend to be powerful already.
When I got to Chernabog, I just paired a fairy/mystic card for each of the attacks (so 2 cards per attack) and defeated him in 3 blows (Medium Mode). Compared to my other Medium fights, I was surprised I had the least number of attacks from Chernabog.
My assumption is that since all of the cards I played were all powered up, and paired with other powered up cards, I was able to defeat him pretty easily. Can't wait to try this on hard!
What tips do you have?