In order to obtain this achievement, you need to find all "common" themed words in a themed round. You do not need to find the obscure words, therefore when you find an obscure themed word it will not count towards your numbers. I think this generally applies more so to digrams but you may come across this occurring in themes also. Themed rounds occur about every 6 games (theme, 2 normal, digram, 2 normal, theme) so it can take some time to actually get a themed round, let alone solve all of the words.
This is a difficult achievement, given that not only are some words you would think fit the theme not included (like "chip" for "casino" or "fire" for "fires"), but there are also words on there that don't seem to fit very well into the themes but are still counted at the end ("red" in "fairy tales"). Basically it will just take a bit of patience to find a theme that you are confident in guessing words for and there hopefully aren't too many words to actually guess.
It seems that for themed rounds the developers usually picked a large word that fits the theme and then the number of smaller words depends on the broadness of the theme as well as the commonality. For instance you for the theme of "school" you may get "scholarships", which takes up most of the letters on the board. Another one was "weightlessness" for the "outer space" theme. So what I would suggest is looking for big words first because they may be the hardest to find, and then work your way down to the smaller words, especially because you can often find smaller words within the big words (i.e. "scholar" in "scholarships" and "weightless" in "weightlessness").
Another key is to be thorough with the endings on your words. sometimes you can knock out 3-4 theme words just by changing the endings on them (i.e. breach, breaches, breached) (cheat, cheating) (plump, plumpish). Plurals seem the most common so if you see an s near your words make sure to use it!
But anyway, to the crux of my solution, I have started compiling lists for the different themes I have encountered. These are obviously not complete, so don't use them as a bible, but they will give you a starting point for the kinds of words you may come across in certain categories, and how you may only have a couple of different "words" but multiple plurals and other endings. These are all straight from the wordament lists so they should all work as words in your rounds. However you may find more/less depending on which particular letters you have and if you have more/less endings (-s, -ed, -er, etc.)
Beverages:
Rum
Buttermilk
Buttermilks
Milks
Latte
Milk
Ale
Ales
Soymilk
Soymilks
Lassi
Tea
Teas
Soda
Sodas
Mead
Meads
Toast
Toasts
Cabernets
Cabernet
Beers
Beer
Tonic
Time:
Wartime
Wartimes
Airtime
Airtimes
Term
Terms
Era
Eras
Timer
Timers
Summer
Summers
Teatime
Beach:
Conch
Conches
Shark
Sharks
Coral
Corals
Rock
Rocks
Bodysurfing
Bodysurfs
Bodysurf
Surfing
Surfs
Surf
Fins
Tide
Tides
Cricket:
Bats
Bowl
Bowls
Bail
Bails
Wicket
Wickets
Keeper
Keepers
Bowler
Bowlers
Overthrows
Overthrow
Covers
Cover
Over
Overs
Shot
Umpires
Stumps
Umpire
Stump
Series
Tea
Tie
Swing
Fine
Tour
Cut
Outswing
Wide
Security:
Breach
Breaches
Breached
Fort
Forts
Force
Forces
Respond
Responds
Responders
Responder
Deadbolts
Deadbolt
Bolts
Bolt
Arms
Arm
Seal
Occupations:
Curator
Decorator
Cleaner
Orator
Aide
Dean
Server
Porter
Poet
Vet
Trooper
Tester
Tutor
Housekeeper
Keeper
Priest
Editor
Host
Cardiologist
Radiologist
Biologist
Pilot
Auto Parts:
Taillights
Taillight
Lights
Rail
Rails
Signal
Signals
Belt
Belts
Boot
Boots
Roof
Throttle
Throttles
Fob
Fobs
Hose
Hoses
Publications:
Copywriter
Copywrite
Writer
Writes
Write
Type
Music:
Choral
Chorale
Noel
Noels
Reel
Reels
Tenor
Crescendo
Chord
Chords
Orchestra
Orchestral
Fairy Tales:
Damsel
Dreams
Red
Bear
Bears
Beds
Bed
Outer Space:
Weightlessness
Weightless
Lenses
Lens
Alien
Aliens
Light
Rotations
Rotation
Stations
Station
Rotate
Ions
Ion
Spheres
Ionosphere
Exosphere
Exospheres
Foods:
Flatbreads
Flatbread
Dals
Breads
Bread
Beans
Bean
Dal
Date
Dates
Wild West:
Packhorses
Packhorse
Posse
Posses
Rope
Ropes
Horse
Horses
Preachers
Preacher
Ranchers
Rancher
Rangers
Ranger
Ranch
Range
Song
Desperados
Desperado
Canteen
Insects:
Fly
Flies
Sawflies
Flea
Fleas
Sawfly
Flit
Flits
Roach
Roaches
Cockroach
Cockroaches
Drone
Drones
Pollinator
Pollinators
Ant
Ants
Nit
Nits
Instruments:
Synthesizers
Synthesizer
Zithers
Zither
“Large”:
Plump
Plumpish
Ample
Super
Grandiose
Grand
Giant
Vast
Monster
Monstrous
Enormous
Extravagant
Extensive
Casino:
Cheat
Cheating
Dice
Hit
Pit
Die
Ace
Rolls
Rake
Roll
Streak
Stake
Rank
Faro
Animal sounds:
Bleat
Bleats
Baa
Baas
Bugle
Bugles
Blub
Blubs
Yowl
Yowls
Booms
Boom
Lows
Low
Moos
Moo
Animal Groups:
Flutter
Clutter
Litter
Herd
Cluster
Drift
Harem
Charm
Earth
Game
Cry
Gam
Rag
Span
Stand
Clan
Clat
Leap
Pace
Nide
Den
Body:
Cell
Cells
Capillaries
Lips
Lip
Iris
Sole
Ear
Nipples
Nipple
Spine
Soles
Tonsil
Tonsils
Shin
Shins
Toe
Toes
Nose
Noses
Math:
Sums
Sum
Set
Sets
Subset
Subsets
Statistics
Statistic
Less
Stats
Stat
Chickens:
Peeps
Peep
Hens
Pens
Hen
Pen
Henhouse
Roost
Rooster
Roosters
Roaster
Pen
Nest
Fruits:
Neems
Neem
Lemons
Lemon
Melons
Melon
Pomelo
Pomelos
Sloe
Sloes
Watermelon
Watermelons
Military:
Paratroopers
Paratrooper
Troopers
Trooper
Troops
Troop
Horses:
Saddle
Rider
Ride
Sidesaddle
Rears
Rear
Sire
Sires
Studhorses
Studhorse
Unhorses
Unhorse
Studs
Stud
Geldings
Gelding
Bridle
Riding
Reins
Rein
Pirates:
Swashbucklers
Swashbuckler
Crew
Seas
Sea
Sail
Sailors
Sailor
Axe
Axes
Housing:
Hospitals
Hospital
Ship
Ships
Flats
Flat
Dorm
Condos
Condominium
Condominiums
Gymnastics:
Handsprings
Handspring
Springs
Spring
Hang
Rings
Ring
Somersaults
Somersault
Scores
Score
Mats
Mat
Trainers
Gainers
Gainer
Trainer
Split
Giant
Spin
Spins
Championship
Championships
Baseball:
Cap
Base
Tag
Bat
Snag
Gap
Caps
Bats
Balls
Bases
Ball
Runs
Run
Lose
Spitball
Bleachers
Bleacher
Liners
Liner
Ace
Weddings:
Bride
Bridesmaids
Bridesmaids
Dress
Tears
Centerpieces
Centerpiece
Rice
Cooking:
Pan
Prepare
Pipes
Pares
Pare
Pipe
Rise
Sear
Housing:
Encampments
Encampment
Inns
Inn
Herb/Spice:
Dill
Mint
Dills
Salt
Nigella
Nigellas
Catmints
Catmint
Sumac
Mints
Salts
Cardamoms
Cardamom
Cardamons
Cardamon
Mace
Family:
Bro
Brother
Brothers
Stepbrother
Stepbrothers
Pet
Pets
Politics:
Referendums
Referendum
Reforms
Reform
Term
Scotland:
Tam
Tams
Burgh
Burghs
Stags
Stag
Pub
School:
Graduate
Graduates
Educate
Educates
Drama
Grads
Smart
Art
Scholarships
Scholarship
Scholar
Scholars
Flowers:
Rose
Roses
Primrose
Primroses
Mimosa
Mimosas
Sea Life:
Whales
Whale
Narwhal
Narwhals
Narwals
Seal
Sole
Eel
Eels
Seahorses
Seahorse
Seals
Swimming:
Lap
Laps
Shallow
Shallows
Splash
Wall
Walls
Gemstones & Jewellery:
Bloodstones
Bloodstone
Gold
Stone
Stones
Ring
Rings
Grind
Bangle
Bangles
Bead
Beads
Lustre
Sunstones
Sunstone
Luster
Netherlands:
Bridge
Bridges
Drawbridge
Drawbridges
Eel
Eels
Seaport
Seaports
Port
Ports
Tart
Oars
Art
Flat
Delftware
Delft
Trade
Delf
Kale
Houseboat
Houseboats
Boat
Boats
Crime:
Fingerprints
Fingerprint
Infringes
Infringe
Stings
Sting
Soccer:
Substitution
Substitutions
Substitutes
Substitute
Subs
Sub
Toed
Toes
Net
Cards
Bars
Card
Cap
Bar
Park
Parks
Back
Kick
Kicks
Backs
Shoots
Post
Shoot
Shots
Mythical Creatures:
Sorcerer
Sorcerers
Siren
Sirens
Orc
Orcs
Roc
Rocs
Oni
Unicorn
Unicorns
Imp
Imps
Wordament:
Challenge
Challenger
Challenges
Challengers
Learn
Race
Digram
Digrams
Weather:
Snow
Snowflake
Snowflakes
Flakes
Flake
Fog
Cold
Cloudbursts
Cloudburst
Cloud
Clouds
Toiletries:
Brush
Brushes
Toothbrush
Toothbrushes
Soap
Soaps
Tissue
Tissues
Shampoo
Shampoos
File
Files
Oil
Oils
Salve
Salves
Chess:
Bishop
Opposition
Position
Bind
Pin
Squares
Sqaure
Queens
Queen
Rank
Color:
Pale
Teal
Tan
Eggplant
Pearl
Pecan
Pea
Land Features:
Glaciers
Glacier
Nesses
Ocean
Rises
Rias
Ness
Rise
Seas
Ria
Sea
Hillsides
Hillside
Hills
Rills
Hill
Rill
Tarn
Hockey:
Skaters
Skater
Skates
Skate
Nets
Net
Line
Lines
Goal
Goals
Goalie
Goalies
Sticks
Stick
Passes
Pass
Score
Creases
Crease
Post
Ice
Facemasks
Facemask
Dekes
Deke
Team
Ref
Circus and Fair:
Tiger
Tigers
Net
Tent
Tents
Stilt
Stilts
At the Library:
Book
Books
ebooks
ebook
Bookshelves
Shelves
American Football:
Players
Replays
Replay
Player
Plays
Play
Long
Ref
Sidelines
Sideline
Screens
Screen
Lines
Line
Computers:
Drivers
Crashes
Driver
Server
Crash
Cases
Case
Blades:
Peel
Tip
Peelers
Peeler
Pares
Peels
Epees
Pare
Peen
Epee
Fires:
Singe
Singes
Ignite
Ignites
Extinguish
Extinguisher
Extinguishers
Torchlights
Torchlight
Alight
Light
Hot
Flight/Flying:
Air
Cruise
Cruised
Undercarriage
Undercarriages
Trees:
Pecan
Elm
Plumerias
Plumeria
Plum
Pear
Sap
Sound:
Ring
Horn
Horns
Echoing
Snoring
Cooing
Snores
Croon
Cries
Snore
Echo
Birds:
Tern
Firebird
Firebirds
Frigatebird
Frigatebirds
Photography:
Flash
Flashes
Flashtube
Flashtubes
Parties:
Farewells
Farewell
Fest
Teas
Tea
Eats
Eat
Happy Hour:
Beers
Beer
Ice
Blenders
Blender
Bender
Blend
Finances:
Salary
Wage
Wages
Salaries
Sells
Sales
Safe
Medieval Warfare:
Arbalests
Arbalest
Castles
Castle
Tar
Farm:
Acre
Acres
Harrows
Harrow
Orchard
Plows
Plow
Crows
Crow
Ewes
Sow
Furniture:
Sofas
Sofa
Seats
Seat
Beds
Bed
Sofabeds
Sofabed
Clothing:
Miniskirts
Miniskirt
Bikinis
Bikinis
Skirts
Skirt
Coffee and Tea:
Percolated
Percolate
Latte
Tall
Pot
Steep
Coffee
Coffeepot
Coffeepots
Pots
Workshop:
Washers
Washer
Shear
Saws
Saw
Rasp
Awls
Awl
Rope
This is all the current information I have. If you wish to add to the list let me know and I will credit you for your contribution. There are several more themes and there are obviously more words per word group, but this is just a rough guide to the kind of words you will get and it may help you find that extra one/two words you're looking for. I will continue adding to this list as I find more words. If you wish to vote negative please provide some constructive criticism :)
EDIT: List updated 01/06/2012, thanks for your contributions Paulfc, Zanzibar McFate, DPC eNeRGy, thirtysmooth, DragonYen, JimmyTheKidd02, Deathisland, Grimmr74, Jackky8586, pmgu2, Adamavich, sider inc, WraithDK, Mattism, Markyshizzle, exiss, SiebeTolsma, Lijnloos, BOB101910 RFP, MCoreBE, FellCracker8, hatchywatchy, DenXD, theonlydragonsi & Gator Dave 86 :)