I'd also have to say Coral Beach Field 2 would be your best option. I've tried farming cards at Mermaid and Phantom but the drop rate seemed low.
Personally, I believe it comes down to your luck.
Sometimes Bad Furies drop out alot of cards, and sometimes they don't.
I usually use two sheeps and make one of them just stand and cast scorching earth while the other one kills normally using aoe skills.