OK, so I've managed to get every medal in the game save for two; the Death Gaze and the Battle Gear challenges.
Now, against Dr. Morroc at his map, it always only produces Neutral cards, so it makes sense that to increase my chances of getting the one additional battle gear Alpha I need this would be the place to do it. But what about Earth cards?
Gemmory is the only character who uses an Earth only deck, but you only encounter him in the cratos maps. And against an additional opponent. You do not earn any more Earth cards than any other battle.
Is the Dr. Morroc stage the only one which offers this advantage of earning only Neutral cards? If I challenge Whirlwind at his stage, do I get more Air cards? (never actually tested this theory. I don't need more air cards).