Regardless if you're talking about Units or Gold, the best answer is: Versus.
Grind those arenas, achieve those milestones, and spend the battle chips on Arena Crystals, from which you can get both Units and Gold (or a Punisher, if you're lucky enough)
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Grind those arenas, achieve those milestones, and spend the battle chips on Arena Crystals, from which you can get both Units and Gold (or a Punisher, if you're lucky enough)