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