Stand in one place (ideally on foot since it's harder to do this on a bike) in the area of the SZ that Chansey is located and pivot around. By simply pivoting in on square and not moving from one square to another, you can get attacked but the steps won't be subtracted from the timer. Wait for a Chansey to appear and start by throwing a ball. If you catch it the first time, cool. If you don't and it doesn't run, try throwing food so it (hopefully) won't run away so fast. You can also throw a rock if you want...anyway just try throwing balls until you in catch it--or until it runs. If it runs...keep trying.