I cant get past Deathstroke

There has to be a way to get past this douchebag. I just cant get past him. I can get the staff from him. then he pulls out the sword and i last a few more minutes but he ultimately kills me everytime

I had the same problem. I've finished the game, and I'd say he's the hardest boss. Took me ages.
My tips for beating him would be to be patient when countering. If you press triangle too quick the counter will fail. That was my problem. If you wait then you should be able to see the opening to counter. I don't know exactly where you're going wrong, but I found that as long as you get the counter right he can't damage you. Then all you have to do is attack.

Alright, just hit him and counter/batclaw pull and hit until u get to the interactive cinematic then counter when the screen says to. Repeat, and remember to throw counter the canister he remote claws at you. And don't get shot either.