Don't fear.

Shuckle confirmed as Mega-Gengar Counter.

Shuckle is OP.

Shuckle is soon to be banned.

Shuckle@Leftovers
Calm
252 HP/ 252 Sp.d/ 6Hp
Rest
Sleep Talk
Accupuncture
Rock Slide

Timid 252 Sp.Atk Gengar can't do more than 30.7%. With 6.25% recovery from leftovers that's 24.45% 5HKO at best.

>Accupuncture until rest is needed.
>Rest
>Sleep Talk either Rock Slide or Accupuncture
>Wake up after 3 turns, rest and repeat until M-Gengar either switches out or dies.

Potential to also have this:

Shuckle@Leftovers
Contrary
Calm
252 HP/ 252 Sp.d/ 6Hp
Rest
Toxic
Shell Smash
Bide

Similar principle.

>Shell Smash until defence and Sp.Def is +6 or rest is needed.
>Rest.
>Wake up
>More Shell Smash
>Bide for Gengar
>Once Gengar is gone continue to sweep with Bide and Rest.