Why is the Deathstroke battle in Arkham Origins so brutal?

I'm pretty early in Arkham Origins and I'm stuck at this part where you fight Deathstroke, and no matter how well I time my counters against him, he always kills me, which makes it impossible to beat him at all, so if anyone has any advice on how to beat him, please say it in the comments if possible. I tried posting this in r/BatmanArkham subreddit, but nobody answered and the post just died there the second it was created, so I'm hoping it'll get any traction at all on this subreddit.