Only retain the gear that you have and sell the rest for extra magic cubes for use in the Factory. Use those cubes to craft legendary gear or for crafting cards if needed.

Use the best possible combinations of gear and cards for custom loadouts. Always use your custom loadouts when completing the single player campaign and never for already completed mines. Consider equipping your characters with explosive blocks cards then using gear that gives you additional bonuses, such as extra explosive blocks or extra metal/precious blocks or bombs. Do not be concerned about any other type of card that increases your earnings, as completing levels is more important.