Chansey is indeed difficult to catch, but this will help to easily catch it.
First bring a Pokemon that knows sweet cent,and go to the part that chansey could possibly appear and use sweet cent,if chansey appear after you use sweet cent it will be more easier to catch than when it appear when you are walking and it won't ran away so easily,so first throw a rock first and it didn't ran than use the ball,hold A until you caught it.
You can also use bait twice then a rock then a pokeball.
This counts for tauros too.
Hope this helped