This achievement is very easy to get right after the origin story when you go to Ostagar with Duncan. I chose Warrior class, so other solutions requiring a Mage did not apply to me.
After meeting Alistair, go back to Duncan and speak to him about the Joining. You will then have a party consisting of Alistair, Daveth, and Jory.
Daveth is the one we want to switch to as our Player Character because he has a Level 1 Poison-Making skill. Now all you have to do is buy one of the required recipes and ingredients for a Level 1 Poison crafting item (either "Venom" or "Deathroot Extract" are both available from the Quartermaster right in Ostagar).
To see which ingredients you need; first make sure that Poison-Making is mapped to Daveth's battle menu shortcuts, then press the appropriate button to bring up the Poison-Making menu. Scroll down to whichever Poison recipe you purchased from the Quartermaster to see which ingredients you need to buy in order to craft an item (1 Toxin Extract and 1 Flask for "Venom"; 1 Deathroot and 1 Flask for "Deathroot Extract").
Now all you have to do is buy the necessary ingredients from the Quartermaster, make sure you are controlling Daveth, and then press the appropriate button on Daveth's battle menu shortcuts to once again bring up the Poison-Making menu.
Scroll down to the "Poison-Making" recipe you previously purchased with the correct ingredients, press the "A" button to Create Item, and once the item is created the achievement will pop.
Quick, easy, and relatively painless as none of the items are terribly expensive, and you can even load a previous save once you have the achievement if you want your money back.
This method is confirmed to work, as I just got the achievement myself this way. I used the "Deathroot Extract" recipe, but the "Venom" recipe should work just as well.