Here is the full list of all 41 OTTTD achievements.
Wage war across 10 separate days
You got an employee to level 99. We're impressed and a little scared
3 star every mission. Show off!
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
Watched the credits to the end without speeding them up
Finish the game
You're making us all look bad. A HEROCORP™ employee reached level 50
At least one skill point on all abilities for a HEROCORP™ employee
Earned total 1 million coins
Double-killed the zombie shark and his buddy
Defeated the Kraken and lived to tell the tale
Earned 42 stars
Play the game 3 days in a row
Nobody likes a show off. A HEROCORP™ employee reached level 20
Unlock all HEROCORP™ employee classes
Fully upgraded the Deathbeamer
Finish a mission with 5 Shield Emitters or 5 Healthenators
Finish a mission without building any towers
Max out a single skill for a HEROCORP™ employee
Spoilt your employee with the most expensive weapon
Earned 21 stars and a free meat tray back at HQ
Well thanks for putting in the effort. A HEROCORP™ employee reached level 10
Revive a HEROCORP™ employee
We shouldn't reward you for letting an employee die. Looks like Steve in HR has some explaining to do
Finish a mission without upgrading any towers
Killed the flying shark or understood our movie reference
Finish a mission by only building rocket towers
First HEROCORP™ employee with over 1337 damage gets a ribbon
Spent 100,000 coins and kept all the receipts for accounting
Bought your first weapon
Give yourself a pat on the back. A HEROCORP™ employee reached level 5
Finish a mission with perfect HQ health
Killed 300 enemies, looked ripped while doing it
First HEROCORP™ employee with over 3000 health gets a discount on their insurance
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