Here is the full list of all 25 Block Heads: Instakill achievements.
Go commit die
Your first kill - with many more to come
Start customizing your character
Start a server
Join a Server
Leave a Server
Play with 15 other people
Win a FFA without dying
Win a TDM without getting a kill
Win a FFA
Win a TDM
Start the level builder
Have more than one player work on a level
Publish a level to the Workshop while no one is connected
Publish a level to the Workshop while someone is connected
Add custom music
Get two kills without dying
Get three kills without dying
Get four kills without dying
Get five kills without dying
Get six kills without dying
Get seven kills without dying
Get eight kills without dying
Get nine kills without dying