The achievement unlocks when you play the entire main story line on hard difficulty. It will not pop immediately following the final battle. There are some final cut scenes first that you can skip and the credits that you can also skip. The achievement will most likely pop on a loading screen as shown in the below video:
The final battle can be simplified if you know its structure beforehand. It is not a conventional fight. You can only damage the boss, Tirnoch, during a very short window after you invoke reckoning mode. Your gauge is empty when you confront the boss. The game provides enemies, "Splinters of Fate" for you to fight in order to charge your gauge.
However, if you bring several potions of fate, you can charge the gauge with the potion. Do so, as soon as the boss stops talking. If you use some potion buffs and an overpowered weapon as shown in the video, you can do substantial damage in one go. On casual difficulty with an OP weapon and lots of buffs, you can zero out the boss's health in one go. The number of rounds depends on the difficulty mode, your weapons, buffs used (or not), etc. Until the boss's health is reduced to zero, after each round, use another potion of fate and, optionally, refresh your offensive buffs. Again, invoke reckoning mode and attack the boss immediately after that.
When you reduce the boss's health to zero, you'll see a dimmed
prompt. Run up to the boss and the prompt will become bright. Press
to deliver the coup de grace. If you fail to act quickly, the boss will recover some health and you'll have to invoke reckoning mode an additional time and respond, again (a little more quickly) to the button prompt.
Watch the end cut scenes and credit (or skip them). The achievement will pop during the credits or on a loading screen after the credits.