The main thing is not to get donked (which is fairly easy with those decks) and drag it into late-game. It's really not that hard with any good metagame deck.
I've been asking myself the same question lately, I'm so sick of my friends deck it donks me all most every game, something that hardly every gets a lone pokemon start...
And before people say that Gyarados does not beat Sablelock, if the Gyarados deck can get even two Karp in the discard, then Gyarados can cause a lot of trouble on to Sablelock thanks to the low HP the Pokemon have. And it certainly is possible.
Use a faster donk deck. Flyphan works, because Sableye has less Hp than Phanpy, and Donphan can hit hard, fast. If the game continues past donking ability, then start sweeping with Flygon.