Kill Gurag the Demigod - achievement

[Infinite Pain]

If I am not mistaken, Gurag is the 1st ACT's boss, right? I killed him like 6 times and even completed the quest but I still have not received the achievement. I suppose that the boss of ACT II (the Reaper guy) also gives an achievement but I haven't received that either and I am almost at the 3rd ARC's boss.

[ANZ]

https://steamcommunity.com/app/269210/discussions/0/615085406662506491/

And I will just remind that almost all the possible questions about the game already have an answers. Just use search function with the key words / phrases next time when you'll have any questions.