Gengar. Or any ghost type in general.
Most Kangaskhan-Mega carry Return, Fake Out, Sucker Punch, and Power-up Punch, while some carry earthquake and seismic toss. Rarely and of them use Shadow Claw.
But, here is the set to counter a typical Mega Kang
Gengar @Black Sludge
252 HP/200 Speed/52 Sp. Atk
Typical Mega-Kang cannot touch Gengar without using sucker punch, so sub up and willowisp is to weaken it ASAP. But also beware of switch-outs
Hope I helped!