You can get up to 4 Life Elixir before recruiting Sylvia the Herbalist. And get 2 maps for the Forger in The Strike Leader mission and The Escape mission.
Once you have all the ingredients use Sylvia to cook a 3x more powerful Life Elixir and use the forger to make two extra copies, if you use Sylvia's cauldron one more time you can make a stronger Elixir again totaling 12 extra life points , enough for Karst to pass the 18 life required for the achievement.
Elixirs are obtained by rescuing the snake twice, capturing Hector alive, and exploring 3 docks before rescuing Volepain's father.