To beat Plox, you should preferably play an attacker which stays in play (no hand disruption) but not as an active pokémon (hand disruption doesn't affect your capability to bring out your attackers).
CurseGengar with Shuckle SW comes to mind as a deck that should have positive matchup if you get 3 energies down (4 Roseannes is a must).
Problem wit tanking with Dialga G Lv.X or Steelix is that Gallade can take them down with one hit, following with Judge and Psychic lock to limit your recovery changes. You should have capability to take down Gallade and follow up with something decent when the Plox begins.
Of course you can also try to outspeed them. There are quite a few speedier decks than Plox, but Spiritomb and Mewtwo Lv.X usually limit them enough. Also should you try to lock 'em up with Sablelock and Initiatives, make sure that they won't ever set up, as only a single Gardevoir can Initiative you every turn for the rest of the game...
Beating plox isn't too simple, as every deck can lose to Giratina/Judge + Plox. Just make your setup consitent and not too dependant on the cards on your hand, as you might experience some troubles trying to draw what you need when powerlocked...