This achievement is fairly straightforward albeit a tad tedious. Once you have a decent gear score, around 2000 or so, queue up for the Prophecy of Madness skirmish. There are two variants to the skirmish both of which end with the Balor spawning at the beginning of Round 3. Killing him will count towards the achievement and you can check your progress using the in game achievement list accessed by viewing your journal. The end medal you get for completing the skirmish does not matter as you only need to kill the Balor. Repeating this 25 times will net you the 20 gamerscore.