Scrafty, preferably with an Assault Vest.
Edit: Didn't see the replace part, you should replace Mega Manectric or Lapras imo, because of Scrafty's arguably better and more solid Intimidate support than Mega Manectric's and because it doesn't use a mega slot (which is better left to Mega Gengar in this team) and it has a better typing than Lapras, considering that its horrible weaknesses to Fairy, Fighting and Flying can be easily supported/covered by its other teammates.
Oh my god... I totally forgot 1 mega per battle. Lmao. I'm using a Perish Song team.
Thanks.