Tips for using flash during outdoor model shooting:
Try to keep the flash light quality soft by using diffuser like soft box, or white umbrella. Try using a gel to change the color of the flash becomes more yellow / golden to resemble sunlight. Try using flash from different angle2 like side-lighting or backlighting. Maximize the power of the flash in order to reduce natural light around, to change the "mood" of the location.
Tips to maximize the use of natural light during outdoor shooting:
In the morning / afternoon, position the model so that the sun becomes backlight. Then use a reflector of 45 degrees in front of the model to reflect sunlight onto the model face as the main lighting. During the day, try to find a shady spot / spot to do shooting. If the sun is very disturbing in the spot you want, try covering the sun with a reflector. This technique is of course only useful if you use a telephoto lens and do close-up photos. Try occasionally to make the sun the main source of light. If it's too hot, try adjusting the model's pose so as not to look directly into the sun.
Hopefully useful