Set a large (10-inch) cast iron skillet over medium heat and add the oil. When hot, add the duck legs, skin side down, and sear until golden brown, about 4-5 minutes. Flip over and cook for another 3-4 minutes. Add 1 cup of the chicken stock to the skillet and bring to a simmer. Cover the skillet with foil and place in a 325 degree oven. After about 30 minutes, remove the foil and continue cooking for another 30 minutes.
Cool until handleable, then cut or pull the meat (and skin) from the bones.
While the duck is braising, make the mole. In a blender jar, add the salsa, pumpkin seeds and epazote. Blend the mixture until velvety smooth, then add it into a small (1- to 2-quart) saucepan set over medium high heat. Simmer for about 15 minutes, then add the remaining 1/2 cup of broth. Season the mixture with salt, about 1/2 teaspoon.
Build your tacos by placing the shredded duck on warm corn tortillas. Spoon on some of the green mole and garnish with the baby arugula.