Caustic Juice is class F so its very common and is tradable. So either you can continually chat with the Mayor, Zim, Bartly, etc. on the map or hunt monsters. Monsters potentially drop key ingredients from any part of their body regardless if you knock it off. Also killing the whole monster only gets you an extra 10%. Here are what Caustic Juice parts look like:
Torso looks like a bunch of horseshoe magnets, (Mech). Arm looks like a green arm cannon and causes explosions, Motar Mark (Mech).
Head looks like hockey masked pyscho (Bio). Body has two criss-cross straps (Bio).
Arm is a purple sleeved withered hand (Alch). Legs are a bunch of rocks floating (Alch).
Seeing how Caustic Juice is evenly split amongst Mech, Bio, and Alch there is no real need to stick around the Wild Woods except for more higher leveled 2nd Ingr. Tour around the Junkstack and Tombstone rise when you need a change of pace.