I haven't played online yet, so this is my first time seeing Deathstroke spam.
What SHOULD he have been doing to get around it? It seemed like he was on the right track, but the guns are just very difficult to get past from full screen.
Block, move in slowly. Block, jump forward block. Okay you are near him. Block and then lantern's might. Okay he's knocked down, block as he constantly wakes up with sword flip. Punish on block. You could also use GL's diagonal air projectile as DS can't cover that spot unless he jump shot.
You see, each character has ways around projectile. If an approach or two causes you to eat sone shot doesn't mean it can't work. You just have to rethink and do it better. Of course you will do mistakes and eat some. It's zoners' game plan to screw with your mind and make you do stupid things.
But it takes time to activate, which gives you time to get in or interrupt him. You just have to keep an eye on him and make sure you don't miss the opportunity. Also once his trait runs out his guns don't work any more, so you're free to get in.