When you first get to Fort Joy after the tutorial on the ship you will be greeted with a dialogue between Dallis, Alexander, and a Lizard. To earn this achievement, you will need to attack Dallis or Alexander during this dialogue sequence. If they have already left, you'll need to start up a new character to go for the achievement. This cannot be earned when you later confront Dallis at the end of Act 1.
There are two cheesy methods for doing this (one with a deathfog barrel and one with what's called barrelmancy). The other alternative is to wait to confront Dallis until you're much higher level though I haven't tried this method. In any case, there is a ladder to the right side of where the characters are talking so use sneaking to avoid talking to the characters and just leave them until you're ready to deal with them. They will stay in the conversation until you deal with them but might disappear once you escape Fort Joy (into the swamps), so maybe don't wait that long for safety.
Barrelmancy is an alternative, game breaking way to play the game where you use a container full of heavy things like fire and water barrels to crush enemies before they engage in combat or even in combat (use the Executor perk to make this your main form of damage in combat). This is done by using the telekinesis skill which lets you move and pick up items of any weight. (You can see more detail here https://www.reddit.com/r/DivinityOriginalSin/comments/747w5x...
). The basic trick is to find a high health or indestructible container. The best containers are certain indestructible ornate chests such as the ones in the Fort Joy Arena (make sure to pick them up while in combat, you won't be able to get them otherwise). But you can also just use any of the backpacks in the game, they just take a bit more precision.
1. Sneak past Dallis while she talks to the Lizard in front of Fort Joy by using the ladder to the right side of the main entrance. Then search around for heavy things.
2. Fill a container/backpack with a many heavy crates, paintings of Alexander, or barrels that you can find. You can select multiple items at once, access options on the container, then select put into. You will have to move it between companions to move your whole party around. Select the container from the inventory and select "send to ..." to allow the character the previously had the container to move. After Act 1 you will get some teleportation items that make this easier.
3. Once you've got enough weight, head back to Dallis in front of Fort Joy (I don't remember how much is needed but you can save before attacking Dallis in case you don't have enough). Sneak as close to Dallis or Alexander as possible. Then select your heavy container and use the throw option to put it next to the enemy you want to kill. This won't hurt them like it did in the original game. Instead, you need to move it from one side of the character to the other, as close as possible. So place it on one side then select it again and select move and place it on the other side. There is an option in the game that turns on circles around enemies this makes it easier to see how close the container needs to be to the enemy (within the circle will hurt/kill them). If you do it right, this will kill the enemy. When used on Dallis or Alexander it should cause Dallis to flee.
Deathfog Method: (May not work any more)
During the tutorial, there is a room marked with a big skull on it that requires a key. In this room are deathfog barrels which will instantly kill you or any NPCs that get caught in the poison cloud. If you are fast and using an undead character, you can grab one or two of these barrels before they all break. Undead characters are immune to the effects. Create a new undead character with the Pack Mule perk and points in sneaking. Grab one of these barrels and take it with you through the tutorial. Once you get to Fort Joy, sneak up as close to Dallis as you can then go to your inventory and choose to throw the deathfog barrel next to you. Attack it and it will kill everyone there. Note that this will kill Magister Borris who is a part of Ifan's quest.
In the comments, Scotty x360a said this method wasn't working when they tried it so it may have been patched out.