Area Words aren't just gained through the story and message boards! You may have noticed after clearing an area that the reward is a new word! The game doesn't tell you how many earnable ones there are, nor where to go to get them. Unlike the campaigns, theres no NPC to report to with stat tracking and no goodies given to you for accumulating these. Its a mindless, pointless extra. However if you wanna claim 100% completion for the game you'll have to be all in on collecting these!
The first thing to keep in mind is the ranking you'll get upon exiting an area! Any rank from B / A / S perfectly good for word unlocking which is generous. You won't need to get every surprise attack, grab every treasure, etc. just be thorough enough! As a matter of fact, you don't even need to fight monsters to get a B rank! Just break every Object, open every Treasure, and collect 30 to 40 (or more to be safe) Chim Spheres!
If you're all about speed this works very quickly and easily in Grass type fields! Because these Area types are really small! Even the largest Grass area is still smaller than any Cave or Temple one. Those are perfectly viable to use though in case you wanna shake things up!
Whatever your Area type of choice is, as long as the level matches that of one where a Word will unlock then you can get one there! I noted ones mandatory to the story, so while in those you may as well work to earn the Word in them? I noted the optional Quest and Abyss areas as well since its pretty likely people will do those to.
L003: Heavenly (story: Peaceful Leading Freedom)
L005: Elegant (story: Chasing Cupid's Phantom)
L011: Halberd (abyss: Choosing Superior Offerings / L011 / Cave
L021: Piling (abyss: Heartless Starting Pilgrim)
L022: Restful (story: Lazy Advice's Exile)
L023: Paling (story: Delicious Past's Weed Eater)
L032: Fattening (quest: Coiling Destiny's Whicker)
L040: Closed (abyss: Entwined Joyous Venom Fang)
L001: Dark Tree
L008: Cat's Eye (story: Great Cursed In-laws)
L009: Tide Road (quest: Essential Idling Old World)
L010: Snow Lamp (quest: Dancing Moruning Masquerade)
L013: Pinwheel (quest: Screaming Returning Princess)
L027: Gold Bird
L047: Life Boat
L048: Foot Stone
L050: Two Wings