You obtain this in the same manner as you do the Golem Graveyard Achievement except instead of defeating 10 Golems you must defeat 25. It is a challenge so it will remain in effect for a limited time and will expire just before Feb 2014.
It is not difficult to get. Any Golem counts towards the achievement whether it is encountered in a story chapter, a bonus mission, or in the arena. They are cumulative across all venues. If you play the story from chapter 1 thru chapter 17, you will encounter more than 25 Golems. If you replay a chapter, the repeat encounters are added to the total.
The challenge pops as soon as you defeat the 25th Golem. The notification is similar to an achievement notification but the word 'challenge' is used instead of 'achievement.'