Here is the full list of all 50 Defentron achievements.
Kill 100 Shields
Kill 100 Regenerators
Finish Motherboard
Kill the split boss
Kill 1000 enemies
Use the overload ability 10 times in the same level
Finish a level with only basic towers (except levels from Mainframe)
Get 3 stars on all levels in the Mainframe
Kill 500 enemies
Kill 100 Fasts
Kill 100 Hards
Kill 250 enemies
Retry a game you won with less than 3 stars
Kill 100 Basics
Finish Mainframe
Kill 100 enemies
Retry a game you lost
Kill 50 enemies
Finish the Matrix
Finish Central Servers
Finish The Dark Web
Get 3 stars on all levels in the Motherboard
Get 3 stars on all levels in the Matrix
Get 3 stars on all levels in the Central Servers
Get 3 stars on all levels in the Dark Web
Finish the game with any stars (not included secrets)
Finish the game with any stars
Finish a level without buying upgrades (except levels from Mainframe)
Finish a level with only Cluster towers (except levels from Mainframe)
Fully upgrade a Railgun power
Fully upgrade a Blaster power
Fully upgrade a Slow tower power
Fully upgrade a Fast Fire tower power
Fully upgrade a Cluster power
Fully upgrade a Chain tower power
Finish the game with all stars
Have one tower of each type at the same time
Kill the Manta boss
Kill the Warper boss
Kill the Worm boss
Kill the Spawner boss
Fully kill 100 Worms
Kill 100 Splits
Kill 100 Swarms
Overload a fully upgraded tower
Encounter every enemy
Unlock every tower
Complete the codex
Discover a secret level