During the course of the game, you will be given the option to choose the mages side or the Templars side numerous times. It is impossible to get both on one playthrough, so pick a side and stick with it. Continue to side with that group until you receive your achievement/trophy.

In Act 1, the missions that affect the outcome are Wayward Son found in Lowtown, Act of Mercy in Gamlens House, and Enemies Among Us in Hightown. The others begin from Showdown which is at the beginning of Act 3 and will lead to the missions Night Terrors, Best Served Cold and On The Loose which can be found from letters on Hawkes desk. If you side with the Mages 5 times, you will receive the Arcane Defender reward. Side with the Templars 5 times and Mage Hunter is yours.