Arcane Defender - 25G
Sided with the mages five times.
Simply make five choices throughout the game that would benefit the mages you come across during certain quests.
*There are only seven quests in which you can make your choice:
Act IWayward Son - Let Feynriel go to the Dalish
Act of Mercy - When talking to Grace, agree to eliminate Thrask (don't really have to once outside, just to let her think you were)
Enemies Among Us - Vouch for Keran to remain as a Templar
Act IINight Terrors - Do not harm Feynriel, support him
Act IIIShowdown - Act III beginning cutscene, simply side with the Mages
On the Loose - Let Emile go free, put his money to good use and flee Kirkwall
Best Served Cold - When speaking to Cullen, select the eliminate them all option
*Credit to FSparacino4 @Xbox360Achievements.org for the quest lists and dialog options.
Alternatively, the opposite will net you the Mage Hunter achievement. May require another playthrough, unless you utilize your saves.