Here is the full list of all 19 PISTOLA achievements.
The base game contains 13 achievements, and there is 1 DLC pack containing 6 achievements.
Kill the total amount of 1000 bats
Kill the total amount of 1000 skeletons
Kill the total amount of 1000 headcrabs
Kill the total amount of 1000 green zombies
Kill the total amount of 1000 golden helmet skeletons
Kill the total amount of 1000 blue zombies
Kill the total amount of 1000 demons
Kill the total amount of 1000 plant zombies
Kill the total amount of 1000 mandrakes
Kill the total amount of 1000 meat zombies
Kill the total amount of 1000 koogle dinosaurs
Kill the total amount of 1000 caveirudos
Kill one unit of each enemy of the current patch
Score 1000 kills for the first time
Score 10,000 kills for the first time
Stop! You already killed 100,000 monsters
Kill the total amount of 1000 Demon's Eye
Kill the total amount of 1000 imps
Kill the total amount of 1000 demon's peashooter