Here is the full list of all 69 Deep Space Waifu: FLAT JUSTICE achievements. It takes around 2-3 hours to unlock all of the achievements on Windows.
Take a photo of a girl
100% MIKA MOKOTO
Kick Justice in the balls
Get the LASER power up
Get the ROCKET LAUNCHER power up
Torn apart your first piece of cloth
Go to your first date
Begin history mode
Defeat DP Defender
Defeat Dickus first form
Defeat DP Miner
Defeat the Fake Wadding
Defeat DP Mothership
Defeat the Thrasher Super Mecha Warrior
Defeat Dickus second form
Defeat Dickus third form
Destroy the Sneak Mothership
Defeat Dickus final form
Finish with the M.I.L.F.
Use the JUSTICE LASER special attack
Get the RAY power up
Get the SHIELD power up
Get the MACHINE GUN power up
Make a 5 STARS stage
Get a special Phone Number
Get a smelling Pantie evidence
Get the F.L.A.T. Multipass badge
Get the super scret spy business card
Reach 10 STARS
Reach 18 STARS
Reach 30 STARS
DATE all girls
100% JULIA SUZUHARA
Complete ONE HAND JULIA
100% NAO NEKOYAMA
Complete ONE HAND NAO
Complete ONE HAND MIKA
Unlock SAYRU-CHAN
Complete SAYRU-CHAN
Complete an ONE HANDED SALE
Unlock SHARP-EYE YUMIKO
100% SHARP-EYE YUMIKO
Complete ONE HANDED SHARP-EYE
Unlock METAL ROCKY-CHAN
100% METAL ROCKY CHAN
Complete ONE HAND METAL ROCKY CHAN
Unlock NORMAL ROCKY-CHAN
Complete NORMAL ROCKY-CHAN
Complete ONE HAND NORMAL ROCKY CHAN
Unlock PUCKY K-9 UNIT
100% PUCKY K-9 UNIT
Complete ONE HAND PUCKY K-9 UNIT
Unlock SERGEANT CHIKARA
100% SERGEANT CHIKARA
Complete ONE HAND SERGEANT CHIKARA
Unlock LIME CODE
100% LIME CODE
Complete ONE HAND LIME CODE
Unlock M.I.L.F.
100% M.I.L.F.
Complete ONE HANDED MILF
Get promoted to the F.L.A.T. SPECIAL FORCES
Infiltrate in the secret TENTACLE organization
Find the PERFECT FLATNESS
Be spotted by the enemy
Explode a mine!
Unlock the SECRET SPY
complete 100% the spy
complete ONE HANDED SPY