252+ Atk Parental Bond Mega Kangaskhan Crunch vs. 252 HP / 252+ Def Cofagrigus: 156-186 (48.7 - 58.1%)
From there, no more of the second hit from mega kanga, which greatly weakens it. Not to mention it should be burned after the first turn.
And if you did happen to let it get up a PuP:
+2 252+ Atk Parental Bond Mega Kangaskhan Crunch vs. 252 HP / 252+ Def Cofagrigus: 309-366 (96.5 - 114.3%)
You're sorta fucked :D
But then again, if you're not packing a sableye, you're pretty fucked either way when a mega kanga is at +2.
However, I'm not sure if after the first crunch mummy immediately activates and you get to avoid the second, or if you have to take both and then mummy kicks in.
+2 252+ Atk burned Mega Kangaskhan Crunch vs. 252 HP / 252+ Def Cofagrigus: 103-122 (32.1 - 38.1%)
Mega kanga ain't getting past cofagrigus when its burned. Ever.
And cof can take pursuit and stuff quite well, so it won't die too easily.
Also:
252+ Atk Kangaskhan (Move 1) vs. 252 HP / 252+ Def Cofagrigus: 22-26 (6.87 - 8.12%) -- 9HKO at best
after which
+1 252+ Atk Parental Bond Mega Kangaskhan Crunch vs. 252 HP / 252+ Def Cofagrigus: 234-276 (73.1 - 86.2%)
Still fails to ko after leftovers. From there you can burn it. You'll die, but mega kanga will have been essentially dealt with.
The only instance where cofagrigus MAY not completely counter is when kang stays scrappy, PuP's to get to +1, then mega evolves and goes for crunch, which btw, cofagrigus still survives and can then burn.
Think of it like this:
252+ Atk Parental Bond Mega Kangaskhan Crunch vs. 252 HP / 252+ Def Cofagrigus: 156-186 (48.7 - 58.1%)
Cofagrigus switches in, takes 51% worst case scenario after leftovers.
The next crunch does
252+ Atk Mega Kangaskhan Crunch vs. 252 HP / 252+ Def Cofagrigus: 104-124 (32.5 - 38.7%)
and then it's burned. so after leftovers, MAX ROLL BOTH TIMES, you're still at 17%
The next crunch does
252+ Atk burned Mega Kangaskhan Crunch vs. 252 HP / 252+ Def Cofagrigus: 52-62 (16.2 - 19.3%)
From there, if you survive, you can just rest up and the kanga is crippled for the rest of the match.
You have to be INCREDIBLY UNLUCKY to lose to mega kanga, even on the switch in.
Now, let's look at best case scenario
48.7+32.5=81.2
81.2-14(leftovers)=67.2
That means you'll be alive with 32.8% to take the burned crunch.
So after the two crunches, you can be anywhere around 17%-32%, and you have to be above 20% to take the incoming burned crunch and survive to get a rest off.
That's pretty darn good odds if you ask me.
You don't exactly have to stay in either, any decently bulky pokemon like garchomp can come in on a burned crunch and start doing stuff while you can save cofagrigus for death fodder.
And did I mention this is SWITCHING IN?