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