Here is the full list of all 29 KILLBUG achievements.
Kill a god
Reflect 666 projectiles
Kill a Tapeworm
Kill a Tapeworm quickly
Kill a Larva Hive with only its projectiles
Destroy 6 spider eggs quickly
Regain 666 health
Stay in the air for a long time
Anger 42 centipedes
Die 13 times
Die to a ladybug
Kill 2 bugs with 1 demon shot
Recover 500% of your health in one run
Kill a bug with the demon shot
Kill 10 bugs in one ground smash
Climb back up after nearly falling into the mist
Fall to your death
Perform a ground smash
Reload your gun 69 times
Recover health after almost dying
Kill a bug with the blood shot
Perform a knife jump
Kill a Larva Hive
Kill 10 bugs in one melee swing
Recover a lot of health from one centipede
Kill a bug with a reflected projectile
Complete the tutorial
Kill a bug with an air dash
Reload your gun when fully empty