SubPunch Golurk forces Chansey out and can KO with Focus Punch if it stays in.
Golurk unfortunately isn't 100% chansey insurance, but its presence on your team does help a lot. I ran a team with 3 physical 3 special pokes that used a Golurk, and didn't have an inordinate amount of trouble with Chansey.
With Golurk you can switch in almost for free (only Toxic stops it), which helps it over other counter such as, say, Heracross (who can't switch in over and over and over). One successful Focus Punch can pretty much put Chansey out of commission, unless it comes in on a very weak special attacker and SR isn't in play. This pretty much forces it to choose between healing itself and trying to heal its teammates, which I find is the only way to reliably beat Chansey. Most pokemon can't afford to switch right in to Chansey often enough to single handedly pull this off, but Golurk can. Luckily, Golurk is strong enough that you can kill a weakened teammate who might be receiving wish. On the other hand, if Chansey switches to something at high health that you can't kill, you can use Substitute and save the hit for a turn with no Wish. You have to predict well, certainly, but it at least has an answer to most of what Chansey can do. Toxic kind of crashes your party, unfortunately, so I'd recommend running Heal Bell somewhere (I did, at any rate).