The Last Straw - p. 9 Dragon Age II Guide
Last update: 11 May 2016
I'd DEFINITELY recommend checking his corpse, as you will find, inter alia, the Staff of Violation and Orsino's Signet Ring. Additionally you will meet Sandal again by the prison exit #2 (M92, 3) and will be able to do the same things as before. Save you game and use the passage (M92, 2) leading to [The Gallows].
Once you get there (M33, 16), Meredith will start acting strange and won't want to believe your good intentions #1. It will turn out that it's caused by the sword made of the lyrium statue found in the Deep Roads #2. The other Templars will also come to that conclusion and they won't fight together with Meredith, so you will have to battle "only" her.
Right after the battle starts, order your men to attack Meredith #1. Just like in the fight with Orsino there will be NPCs helping you in the battle. Look after the health of those attacking Meredith, as her attack will cause massive damage. At the same time look out for her other attack, including a charge that knocks down anyone on her path #2.
As you deprive Meredith of around 1 of her health, a short cutscene will play #1. Meredith will jump onto a platform where you won't be able to attack her. It doesn't mean the end of trouble, as she will summon a Gate Guardian to help #2.
The Guardian is best fought from a distance #1. If you plan on direct combat, you must remember that his attacks will take lots of health from your warrior and rogues #2. In such situations you must quickly react, as you cannot allow anyone from your team to die.
Defeating the Gate Guardian will lead Meredith to return onto the courtyard #1. Start attacking her again. This time Meredith will run from the courtyard after losing around half of her total health. Be ready to fight another Gate Guardian #2. Just like before, be very cautious and try to attack him most of all from distance.
Defeating the Gate Guardian will let you advance to the hardest part of the battle, as apart from Meredith, lesser Stone Statues will also keep appearing #1. I'd definitely recommend focusing on them #2. Meredith in the meantime should be focused on fighting the NPCs and you can't allow too many Statues to appear.
The Statues will be coming from various sides, so it's worth to stay cautious and keep checking the map #1. Attack Meredith only during short breaks between fighting the Statues. Eventually there should be no Statues left. Only then should you focus fully on damaging Meredith #2.
After depriving Meredith of another part of her health bar, two new Gate Guardians will appear at the same time #1. This time the monster will be very good at attacking from a distance, so try to fight them directly #2. Of course attack one at a time, keeping an eye on your party's health.
Eliminating both Guardians will let you advance to the last part of the fight with Meredith #1, which shouldn't be interrupted by any cutscenes with new monsters. Keep attacking Meredith until her health bar is almost empty #2.
A cutscene showing Meredith's death should play #1. You can sit back and enjoy the game ending #2. Congratulations!