The Witcher 2: Assassins of Kings

This missable achievement occurs on Roche's Path towards the end of Act II, after you return from the mist and reenact the witch's burning. Shortly after, you will see the King at 11 PM, have a fight, and then meet Deathmold at the tent across from the inn. He will offer to send you on Hanmarvyn's Blue Dream, which is a necromantic ritual which will allow you to experience a memory through someone else's eyes.

Before you start this mission, Deathmold will tell you to procure the Gadwall potion. Master Myron is a merchant who is standing in front of the tent, so just walk outside, buy the formula from Master Myron, make the potion (it requires 1 each of Rebis, Vermilion, Caelum, Sol and Aether), and re-enter the tent.

Right before you drink the potion and speak to Deathmold, STOP AND MAKE MANUAL SAVE #1. There are "points of no return" in this mission, and if you go too far, you permanently miss this achievement. A backup save will prevent this. Then drink the potion, talk to Deathmold, and start the mission.

1. When you start this mission you take over the memories of Auckes. At first, you and your companion, Serrit, will run through the mountainside killing harpies. This will fade into the next section.

2. In section 2, follow Serrit closely and avoid traps. If you hit a trap, you will be kicked out of the memory and will have to restart. Serrit walks exactly where you need to, so if you stay directly behind him you'll be fine.

3. You will be in the camp. STOP AND MAKE MANUAL SAVE #2, in case you don't get this right. There will be two guards in front of you. One will go right and disappear, the other will go forward. As this guard walked forward, I walked up and hid behind the first tent on the right. He stopped, looked behind him, and continued. I then moved up between the first and second tent on the right. The guard doubled back and walked right past me, and then I continued down the path and made a right. This takes you to the next sequence.

4. MAKE MANUAL SAVE #3 RIGHT WHEN THIS STARTS. THIS IS THE POINT OF NO RETURN. For this section you walk behind Serrit for 10 seconds. That's it. And then you go to section 5...

5. You're going to fight a half dozen soldiers here. If you die, you get kicked out of the memory and will miss out on the achievement (unless you reload one of the backup saves I asked you to make). The soldiers aren't any tougher than the ones you've already fought, so play to your strengths and take them out. THE ACHIEVEMENT POPS AFTER THIS SECTION.

6. You already have the achievement, and the final fight is just for fun. It doesn't end well.

If you have any questions or if I need to elaborate, please ask in the comments.

"Necromancer"
Relive all of Auckes's memories in Dethmold's vision.

This is relatively simple, but very easy to miss due to the nature of the quest. You will first need to side with Roche and complete the various quests up until the point of Dethmold's ritual. This is part of the "Assassin of Kings" questline, so you can't really miss it. Once you're ready to enter the assassin's memories, drink the Gadwall potion that you need to make. You can buy all the ingredients and the script to craft this item from the merchant just outside the medical tent.

Master Myron sells everything you need to craft this potion, but the names don't appear quite the same in his inventory as they do on the alchemy crafting scree.

You will need 1 of each of these items to craft the potion:

Gadwall Script
Rebis (Celandine)
Vermilion (Wolfsbane)
Caelum (Green mold)
Sol (Verbena)
Aether (Scleroderm)

After crafting this potion you can then re-enter the tent, drink the potion and enter the memories of the assassin. From here on you need to complete the dream without making a mistake or dying.

I recommend that you make a manual save just before you enter the cave with all the traps. Just in case you make a mistake and caught in one as this will end the memory. Follow all the instructions you're given. If you make one mistake in the dream it will end and you will miss the achievement.

You can make an optional save here if you think you may die in the final fight it's a good place to revert back to.

After you complete the cave, you will soon find yourself in the camp, allow the first guards to pass, then kill the patrolling guard once he has his back turned. This is relatively easy, but if you run ahead and make it obvious you're there, the memory will end.

The final area requires you to fight off a group of guards. At this point you cannot die, but if you the memory will end. This fight isn't hard so you shouldn't run into any issues. But you can revert back to your save if you made one in the previous section. Once the guards are dead and the achievement will unlock. From here on out you can do whatever you want in the final fight.

The following video will walk you through the quest after making the Gadwall potion.