Pikachu315111
Ranting & Raving!
Nope, if you haven't caught the Shadow Togetic but defeated Fein you just need to leave Outskirt Stand, come back in, and he'll be right there to challenge again (and he'll also have different dialogue).The only Shadow Pokémon who cannot be re-snagged is Togetic. You can only fight it once, in the post-game, so be sure not to launch any really strong attacks at it, or save before the battle with your impostor.
HOWEVER there is a major softlocking glitch you should be aware of: if you catch his Togetic but lose the battle not only will he not appear again (thus you lose getting the Togetic) but also everyone in the game still hates Wes. Obviously you can fix this by just reloading the save file... unless you save during this state so now the only thing you can do now is restart the entire game. So, yeah, don't catch the Shadow Togetic and then lose.