All possible options please ;w;
You could either use Taunt or Imprison + Memento. The Sinistea line, among others, get both Imprison and Memento.
As SadisticMystic points out, Substitute also prevents the opponent from fainting itself to Memento.
The simple method is to put up a Substitute. Memento fails against that to the point where it prevents the user from fainting.