Act of Mercy: When confronting Grace, turn her and the other mages over to the Circle. You must do the same when confronting Ser Karras (or the game bugs : acts like you killed Ser Karras and your decision does not count toward the mage hunter achievement).
Best Served Cold: Agree with Samson that the mages need to be dealt with. When Samson returns with Cullen and the Templar reinforcements, choose: "He's one of Grace's" followed by "Kill them all."(Also works with "Give Samson another chance" Xbox and PS3 confirmed.) Finally give Meredith the names of those involved.
With careful balancing of decisions, it is possible to obtain both achievements with a single file, reloaded only as far back as right before the Arishok fight. In Act 1, pick (2) options in favor of one faction and the third in favor of the other, and then in Act 2 during Night Terrors, either allow Feynriel to go to Tevinter or make him Tranquil depending on who you sided with once in Act 1. This will create a 2-2 split with exactly 3 more decisions to make in Act 3. This will however require going through the Arishok a second time, as the third decision must be made before you get another opportunity to save, at which point you are committed to a fourth and fifth in favor of that faction to unlock the achievement.
Siding with Orsino or Meredith in Showdown at the opening of Act 3 causes the decision at the end of Best Served Cold to not count if you side with the opposite faction. (Agreeing with Samson and Cullen will not count toward Mage Hunter if you sided with Orsino in Showdown, and vice versa.) This is likely tied to who gives you the quest. It is uncertain whether this is a bug or intentional.
Tested on PS3.
Confirmed on X360 if selected the neutral response (side with neither) during Showdown.