To be completely honest, I do not think that Imposter is broken at all in the current metagame, as people have been saying. Chansey is definitely something that one needs to consider while making a team, but it isn't very hard to make a team Chansey-proof. A while ago, in order to do this, I had all of my sweepers walled by Shedinja, and when Chansey switched in, I would switch in Shedinja. If the opponent had a Shedinja as well, I would use my Mold Breaker Pursuit Giratina. With a team like that, you could have 4-5 (5 if you have an offensive mold breaker with Pursuit) offensive members and a Shedinja, and you'd still be Chansey proof. You would not have to consider making your individual sets Chansey proof, as your entire team would be because of how it was designed. Now, of course, when I ran that style of team, I did not have 4-5 offensive members, and instead had 2 offense, 2 defense, Gira, and Shed.
Now, I've been using a different strategy for walling my team. I run balanced offense/defense, so I generally have one or two bulky pivots that have utility for my team. Anyways, I realized that I could run sweepers that could destroy large amounts of the metagame with ease, but still be walled by these two bulky pivots.
My most recent "good" team (e.g. not my gimmick teams) has a Protean Mewtwo Mega Y with the moveset Volt Switch, Moonblast, Steam Eruption, and Switcheroo. It has Choice Specs. This set is incredibly strong, capable of destroying many bulky Pokemon, especially OHKOing Scizor Mega with Specs Steam Eruption. In order to wall this, I use a Pokemon that I would have on my team anyways, even if Chansey didn't exist. I run Magic Bounce Venusaur-Mega with Venusaurite. None of the attacks do very much damage to Venusaur, as it is a bulky Pokemon that resists all of its coverage. It's also not common enough in the current metagame that I have a problem with my Mewtwo being walled. And just in case there is another Venusaur-Mega, I have something capable of killing that.
My other sweeper is a Protean Slaking (this sounds like an odd choice for Protean but bear with me) with the moveset Shadow Sneak, Extreme Speed, V-Create, and Thousand Arrows. Now I use Slaking because this is a Gengar-Mega check, switching into Judgment, then Slaking uses Shadow Sneak, doing a lot of damage and becoming immune to the move that comes next, Secret Sword. Then, it uses Extreme Speed to become immune to Judgment, and then Shadow Sneak/Thousand Arrows to solidify the kill. V-Create is just a powerful move that destroys anything that doesn't resist it, for the most part. V-Create + Extreme Speed KOs Yveltal. I wall this with another thing that I have on my team anyways, a Flash Fire Scizor-Mega. It is bulky, a terrific Kyu-B switchin, and a perfect counter to my set. A Chansey would need to crit twice when I was at 75% in order to kill me (IIRC). In the mean time, I can boost my attack with Meteor Mash (hopefully), and Roost/King's Shield my way to victory.
My last two members are Chanseys, because she is still an incredibly useful Pokemon for scouting, and I counter sweep if I get the chance (meaning the opponent hasn't taken time to make their team Chansey-proof like I have).