Heat oil in a large skillet over medium heat. Add garlic and cook, stirring often, until beginning to brown, about 2 minutes. Add red pepper flakes and cook, stirring, until fragrant, about 1 minute. Add greens a handful at a time, tossing until wilted between additions; season with salt and pepper and cook until all greens are wilted and softened, about 3 minutes longer. Add olives and lemon juice and toss to combine; season with red pepper flakes and more lemon juice, if desired.