Act of Mercy - p. 3 Dragon Age II Guide
Last update: 11 May 2016
Step 4, Option C: Promise to help the apostates get out at all costs
If you intend to guarantee safety to the apostates at all costs, you should choose the lower right dialogue option at the end of the conversation with Grace #1 (it will result in, inter alia, worsening the relations with Aveline and Varric). Luckily you don't have to worry about Thrask dying, as even in the worst case scenario nothing will happen to him. Don't leave this location at once. It's worth to explore the caves as well as check the corpse of Decimus #2, beside whom you will find a couple precious items.
During the way back to the surface there will be two fights, but unlike the two other options you won't be fighting Corpses but a bit harder Templars and Templar Archers. The first fight will take place where you met the running apostate #1 and the second beside the exit #2 and this time you will also have to face a Templar Lieutenant. After taking care of the enemies, go out of the cave (M38, 1) and return to [Wounded Coast Approach] (M37, 2)
After getting out to the surface, a conversation between Thrask and Karras will automatically start #1. Unfortunately you won't be able to choose any dialogue options, as the fight with Karras and his men will begin immediately #2.
Defeating Karras isn't easy, but I'd suggest taking care of him in the first place #1. If the Slaver will bother you #2 you can devote some time to get rid of him.
Only after eliminating Karras should you take care of the Templars and Templar Archers #1, occupying both paths leading to the caves. After the battle the game will automatically switch to a conversation with Grace and Thrask #2. Irrespectively of the chosen dialogue option you will receive thanks from Grace together with Alchemist's Protective Handguards. Don't forget to examine Karras' corpse, as he was using an excellent sword, shield and mail.