This Achievement is missable! If you miss it, you must start a new Game or higher Difficulty and play again to Deathstroke.
For the success you have counter Deathstroke normal Attacks and in the QTEs (Quick Time Events), press the Counterbutton to the right time (not too early or too late)
If you fail a counter in the Bossfight, restart over the Menu (Start Button) a new Try. The Fight has 3 Phases, in Phase 1 you have in the QTE 1 Counterevent, in Phase 2, 2 and in Phase 3, 2-3. (Keep in mind, dont forget to counter his normal attacks outside the QTEs)
Between the Phases is another QTE, here you must press the Counterbutton very fast. (not sure is this towards to the Achievement because "Single Counter" in the Achievementdescription)
In the Phases is also a QTE, here you have counter the throwing Barrel first with the Counterbutton and then throw it with LT+Y to Deathstroke.