Your fight against Deathstroke in Batman: Arkham Origins is a fairly straightforward battle because most of it is Quick Time Events (QTEs). You don’t need to follow any actual strategy to take him down.
Your fight is broken down into three main parts. In the first two parts of this fight, you’ll mostly be waiting for him to attack so you can counter and get some damage onto him.
Over the course of this fight, Deathstroke may choose to throw a flash grenade onto you. If this happens, repeatedly tap the counter button to block his damage and then fire back at him with a beat down.
Other than this flash grenade, Deathstroke can also fire his remote grapple towards you, if you happen to be in this scenario, you’ll need to wait for the counter prompt and tap the counter button with precise timing.
If you time the counter for his remote grapple properly, you’ll send a barrel flying at him, and then you can close the gap between you by using Evade and wait for another counter opportunity.
Note: Cape stuns are ineffective against Deathstroke, you can hit long combos on him if you time your counters properly but don’t get too aggressive as you might give him an opening to hit back at you.
Near the end of this fight, Deathstroke will draw his sword and when this happens, he becomes a much bigger threat, but you can take him out if you keep playing to the same strategy of countering whenever prompted to.
Eventually, if you keep your timings precise, Deathstroke will fall. It’s a slow battle and frankly; you can’t do anything about it. You will have to play it in such a way if you want to defeat him.
Batman: Arkham Origins apart from its main storyline offers you side-quests in which you are required to find collectibles like Enigma Datapacks and Anarky Tags. If you are not fond of collectibles, you can take on the task of destroying GCR Comms. All these activities count towards 100% completion!