The solution given above is absolutely correct, but if you're like me and you don't want to play through the entire game a second time for one achievement, then you could do what I did:
Wayward son: Side with the mages
Act of Mercy: Side with the templars
Enemies among us: Side with the mages
Night Terrors: Side with the templars
Save your game right before the battle with the Arishok
Showdown: Side with the templars
On the Loose: Side with the templars
Best Served Cold: Side with the templars
After you report back to Meredith, the achievement will pop. Now reload and get your Arcane Defender Achievement.
As long as choose the templars' side twice between Act 1 and 2, as well as choose the mages' side twice, and then 3 times in Act 3, you could get both achievements in about 30 minutes. Worked for me!