All information is extracted from this video, which made all the research necessary, give them a view, you will probably need it anyways, my guide just shortens what you exactly need to do.

Basically there is a list of NPCs, if any of the NPCs die, you fail the achievement. After you finish the game, you get the achievement.

The list of NPCs is:

• Alfira
  
• Arabella
  
• Bex
  
• Cal
  
• Cerys
  
• Dammon
  
• Danis
  
• Ide
  
• Lakrissa
  
• Lia
  
• Mol
  
• Rolan
  
• Umi
  
• Zevlor
  
• Zorru

Some of the NPCs are in no danger unless you go for some very bad choices, some are in mild danger if you do a combat, that may include them, some of them are avoidable via dialog, and all of them are avoidable, if you check out the video. I am going to describe actions in each act that you need to take, if you need details, go to the video.

You need to convince Rolan to stay in the camp.

Save Arabella from the snake.

Unless you are doing Dark Urge, no actions needed. If you're playing The Dark Urge, you need to knock her out before the cutscene to get a replacement NPC. There are plenty of guides on how to do this.

Take out the goblin leaders, duh. Preferably without them going to the Refugee camp.

You don't need them, they may die and it doesn't fail the achievement, check out the NPC list or preferably the video!

Try to avoid it. If you really want to do it, protect mainly Cerys, and avoid AoEs. Some of the NPCs may be in the area, but if you avoid AoEs, they should be fine, for more details check out the video!

You need to meet Rolan at Last Light Inn and then rescue him from the wraiths. For minimal risk route, check out the video.

Duh, you need to save them via the back route and get them to Last Light Inn. If you don't know this yet, you need to do this before the Shar Temple.

You need to encounter Arabella, and tell her that you will find your parents to get her into your camp. Do not tell her that they are dead.

You don't need to rescue him from the Moonrise Towers. You can, but you don't need to. The dead Zevlor in Act 3 is different from the Zevlor that needs to survive.

He must not die during the Dame Aylin quest. If you want to do this quest and do it on Honour Mode, check out the video for the best routes on how to keep him alive.

Just don't let anyone die. Don't summon critical NPCs into the final battle preferably.

That's it? I mean what about <insert another random NPC>. NO, you don't need them.

You don't. You can choose any ending you like.

I've personally tested this, and used the ending, where Gale sacrifices himself. You need to convince him to do it at the stem.