Here is the full list of all 41 OTTTD achievements.
Finish a mission with <10% HQ health. Good luck filling out the paperwork back at HEROCORP™
Max out a single skill for a HEROCORP™ employee
Finish a mission by only building rocket towers
First HEROCORP™ employee with over 1337 damage gets a ribbon
Earned 21 stars and a free meat tray back at HQ
Killed the flying shark or understood our movie reference
Spent 100,000 coins and kept all the receipts for accounting
Finish a mission without upgrading any towers
Revive a HEROCORP™ employee
Play the game 3 days in a row
Well thanks for putting in the effort. A HEROCORP™ employee reached level 10
First HEROCORP™ employee with over 3000 health gets a discount on their insurance
We shouldn't reward you for letting an employee die. Looks like Steve in HR has some explaining to do
Finish a mission with perfect HQ health
Give yourself a pat on the back. A HEROCORP™ employee reached level 5
Killed 300 enemies, looked ripped while doing it
Bought your first weapon
Earned total 10,000 coins
Completed HEROCORP™ training
Earned 5 achievements
Fully upgraded a tower
Earned first 3 star rating on a mission
You earned your first level up
Unlock all HEROCORP™ employee classes
At least one skill point on all abilities for a HEROCORP™ employee
Nobody likes a show off. A HEROCORP™ employee reached level 20
You're making us all look bad. A HEROCORP™ employee reached level 50
You got an employee to level 99. We're impressed and a little scared
Earned total 1 million coins
Spoilt your employee with the most expensive weapon
Defeated the Kraken and lived to tell the tale
Double-killed the zombie shark and his buddy
Finish a mission without building any towers
Fully upgraded the Deathbeamer
Finish a mission with 5 Shield Emitters or 5 Healthenators
Finish the game
Earned 42 stars
3 star every mission. Show off!
Killed 9001 enemies with little to no remorse except for that one guy... he was cool
Watched the credits to the end without speeding them up
Wage war across 10 separate days