I'm hardly an expert but the most successful I've been so far is using Grabbix with and without variations, as mentioned in the other thread here. Basically, something really fast. I believe grinding the silver league makes more money than grinding AI3 even if your win rate is far lower, below 50% in comparison to next to 100%. But just catching one old deck (I've even seen some with just 1 HP!) and getting an easy elemental master can net you 200+ electrums. Slow decks might be more successful but take longer, of course.
