I actually think Mime Jr is a very viable way to win by lost world. If your opponent is teching for Gengar Prime, like most people will, then Gengar Prime will be one shot. So if you're going to have a Pokemon be one shot, why go through all the trouble of setting up Gengar when you can do it quickly via Mime Jr? It's a fair question.
You can use him in combination with cards like Judge or Looker's, which force your opponent to shuffle their hand into their deck and give you a fresh new top 4 cards in their deck to get rid of, as well as send any Pokemon they were accumulating back into the deck.
As for techs, you could, as has been mentioned, run Mesprits to spot SP dead in its track. You could also tech in the typical Honchkrow or Palkia X to get Pokemon they try to hide in their bench or discard pile. It's a neat idea for sure.