...I am admittedly extremely new here, but I'm fairly certain that if you let Mega Kangaskhan get enough boosts that Lugia or Giratina couldn't wall it, then it's your own fault if you get swept. I'm not denying that Mega Kangaskhan is really good, especially with Power-up Punch, but like I mentioned, status should be a killer for it. Gliscor, Skarm, and Ferrothorn might be great physical walls, but Gliscor doesn't usually carry status, and Skarm and Ferrothorn are weak to Power-up Punch, which means, yes, they're not suited for dealing with Mega Kangaskhan. But what about Sableye with priority WoW? Or Resttalk Spiritomb with WoW? Or pretty much anything with WoW? Even the likes of SubDisable Gengar should also be fairly effective.
Healing Wish is getting popular? What even runs that in OU, Cresselia?