Have any doubts about Kanghaskan’s usefulness? Take a look at this video and see for yourself the complete beast that he now is in Pokemon X & Y. With the right moveset, he can easily set up and sweep an entire team. I have many more recorded battles (most against Stephane also) that end up with the same result.
However, he does have some weaknesses so he isn’t necessarily OP although I’m sure many would argue otherwise. This set is jolly with maxed out Attack and Speed with the remaining EV’s thrown into HP. He carries Dizzy Punch (51% chance of confusion, 70 base attack), Sucker Punch (Attacks first if opponent uses an attack, double damage with mega evolution), Fake Out (Free near 50% damage to start a battle) and Power-Up Punch (Base Power only 20, but double attack increase every turn because of mega evolution).
My strategy against those are to take them out quickly and not let them set up. Status moves (especially burn) are crucial too. They can still be taken down easily with one hit, but make sure not to let them boost their stats.