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