Curse you Kangaskan...; - ;

I like to use rocky helmet intimidate Stoutland to counter it. Getting the attack lowered+double RH damage is great to watch. Then Stoutland can come back with a solid hit, or you can try to predict a switch and cripple whatever comes in with Thunder Wave.

 

Isn't this Mega Pokémon banned from OU or something? It can be very annoying but maybe you want a Ferrothorn, give it Rocky Helmet (As suggested) and Iron Barbs. If will be dealing a lot of damage, and if it survives the first barrage of attacks, it will be dealing out even more. This may give you time to use a move like Power Whip if you desperately want it dead, but I recommend setting up your stealth rocks/spikes, or maybe even giving it leech seed then protect to give it even more damage. Then, after all the damage it has taken, send in a Pokémon with a priority STAB move to kill it! Should do the trick. They may switch out, but so what? They will get damage from any hazards you set up, and if you let your priority Poke survive, it will be the last switch that Pokémon makes.

 

It is banned from OU on Pokémon Online and Pokémon Showdown but in the WiFi community it is still allowed in things like Battle Spot since Game Freak doesn't follow tier lists and allows pretty much everything :/

Rocky Helmet Ferrothorn is usually the best check to it since it take so much damage from RH + Iron Barbs damage. Rocky Helmet Skarmory also walls it to no end since it can set up hazards to deal damage each time it switches in, whirlwind it out if it has a boost, and instant recover any damage with Roost.

 

I can't blame them though, it's kinda weird for a the Pokémon company to follow some fan made rules about their game. They have the VGC rulesets.

 

You are more than welcome @[member="Myuuriko"] ^^

If you are looking for something not so standard then Cofagrigus is also a pretty decent check to Mega Kanga. Once it is mega it no longer has Scrappy and Cofagrigus will be immune to Fake Out, Return, and PoP (Power Up Punch) while taking very little damage from Earthquake if fully invested in Def. That leaves only Sucker Punch able to hit it either forcing it to switch out or allowing you to burn it with WoW (Will-O-Wisp) as Sucker Punch fails.

252+ Atk Parental Bond Mega Kangaskhan Earthquake vs. 252 HP / 252+ Def Cofagrigus: 97-115 (30.3 - 35.9%) -- guarenteed 4HKO after Leftovers recovery

If for some reason it does have an attack to hit Cofagrigus then it will gain its Mummy ability instead of Parental Bond making it only attack once and becomes much easier to check as well.

 

In my case, my Gliscor does well against Mega Kangaskan~ ^u^ All you have to do it to make sure your Gliscor gets poisonned with its Toxic Orb (for healing) and you protect yourself...after using Toxic against her and her baby! ;D Then you protect until it dies 'v'

It's kind of slow, but it works! I heard Rocky Hemlet Mandibuzz is pretty good too. All defensive Pokémon are good actually. 'v'

And if you plan to use Gengar (not Mega Gengar), you can always use Will-O-Wisp to burn it...then Sucker Punch won't be as effective? And, like everyone said, Rocky Hemlet Ferrothorn is Mega Kangaskan's worst nightmare~ ;3

 

.....I mentioned Sucker Punch when I suggested Cofagrigus and since it only even works if you attack you can simply Will-O-Wisp it making it useless since at that point even Earthquake will only be doing around 15-18%