Here is the full list of all 35 Beeftacular achievements.
Die and ragequit
Play for 2 hours straight
Finish all 90 official classic levels
Finish the first 45 official classic levels
Be fast
Do a ranged triple kill
Do a ranged double kill
Kill a target using a projectile while falling
Kill a target in front and back of you in one jump
Create a nice, custom level in the editor
Create 10 custom levels
Publish 15 nice custom levels in the Steam Workshop
Play a custom level
Complete 10 custom levels that were created by other players
Complete 100 custom levels that were created by other players
Kill a target that is not on screen
High speed crash against a wall
Kill all targets of a level using projectiles only
Get more than 10 ranged kills in one level
Take an interested look at the credits
Complete a secret level
Complete a level with less than a second of time left
Less than one second difference to the current world record
Kill a target by shooting a projectile through a portal
Kill a target using a falling wall
Find a secret in a level
Complete a speedrun
Do more than 15 consecutive walljumps
Get a time over
Get cut in half by a crossbow bolt
Jump through a laserbeam where you can still feel the heat
Create a big custom level
Destroy a destructible wall using your body only
Do three double kills in one level
Complete 100 speedruns