Recently thought of one. It's how to legitimately get a Dark-Ghost Shedinja in battle. Note that I didn't come up with the whole thing, just improved it to be possible.
You need: 1) Shedinja with high speed and Focus Sash, and Mimic, which can be tutored in Gen III
2) Starmie with Protect and Mirror Coat
3) Something with Tailwind that can get it off then faint in the same turn
4) Must be Double Battle
Send out Starmie and the Tailwind Pokémon. Set up Tailwind, and have it faint in the first turn. Have Starmie Protect so it doesn't faint just in case. Send in Shedinja, and have it use Mimic on Starmie, who uses Reflect Type on anyone (If you don't know, that move changes the user into the target's type(s)). If Starmie faints, switch into Sableye or Spiritomb. If not, switch it next turn. Focus Sash will keep Shedinja alive if attacked. On the next turn with one of the two out, have Shedinja use Mimic on it, which will turn it into Dark-Ghost. You take it from there.
It's not perfect, but it can work.