If i knock you the sf gengar with the arceus curse gengar's attack shadow skip while bring back the curse gangar to my bench would fainting spell effect the curse gengar or the new active pokemon.
That means that hit-and-run attacks will make Fainting Spell affect the proper target. Similarly, if the Attacking Pokemon leaves play (such as Uxie's Psychic Restore), Gengar targets a Pokemon that is no longer in play, and it can't return the knockout.* Gengar is receiving errata for its Poke-POWER, "Fainting Spell". Fainting Spell affects your opponent's Attacking Pokemon, not your Defending Pokemon. The correct card text should read, "Once during your opponent's turn, if Gengar would be Knocked Out by damage from an attack, you may flip a coin. If heads, the Attacking Pokémon is Knocked Out." (Nov 7, 2008 Pokemon Organized Play News)