I had this same problem and came to Serebii Forums with this question like you. One piece of advice that helped me a lot was DO NOT EVER USE BAIT OR ROCKS!!! Just throw Safari Balls as many times as the Chansey allows you. Trust me, it works after a while. I know own three Chansey. On my third, I got Lucky Egg! I also saw a shiny Rhyhorn in Safari Zone... which really scared me since there was a very small chance I would catch it... but I luckily did on my first Safari Ball. Now it's a shiny rhydon. Hmm... well, I went off topic, sorry about that. Anyway, I don't know how to explain areas in Safari Zone or I would tell you which area I encountered Chansey the most.