Finn is (in Norah's words, a garden gnome) and has very strong lightning, fire and water elemental magic. He will be avaliable to your party at the beginning of chapter 4 where he will ask you to help him go down a well and help the people of the town. When he joins the party the achievement may not pop, the reason for this though is that you must finish his quest to unlock it. Doing this means you must follow the path Untill you come to the boss fight of the well. Make sure to use thunder on the blue snake, water on the red snake and for some reason, the green snake doesn't seem to have a resistance so make sure you use igniculas on that one specifically as it can shoot venom and stun you leaving you open for more attacks.