Well, ten Elnoyle cards to get an Energy Crystal. Or, high level Grendels to get Laser Cannons (you can also get them during the Laguna dream sequence from high level Elastoids, I believe, but that isn't very efficient). You need Blitzes to get the Power Generators, and you can't get them until you get the mobile Garden.

There might be cards which can refine to Laser Cannons, but I can't remember.