Heat the oil in a large skillet over medium heat. Add the garlic and cook, stirring constantly, for 2 minutes. Add a few handfuls of the greens, allow them to wilt slightly, then add more greens in the same manner. Toss well to coat with oil.
Add the soy sauce, fish sauce and sugar; toss to combine.
Raise the heat to medium-high and cook until the water released by the greens mostly evaporates, about 5 minutes. The greens are done when a little liquid still clings to the leaves and the stems are tender. Season with salt and black pepper to taste, sprinkle with chilies and serve.