Here is the full list of all 25 The Showdown Effect achievements.
Score 20 kills with the Machinegun
Throw an item on the ground without hitting anything first
Score 20 kills with the Pistol
Score 20 kills with the Shotgun
Score 88 kills with the Katana
Score 20 kills with the SMG
Score 10 kills in one match with the Pistol
Score 10 kills in one match with the Shotgun
Score 10 kills in one match with the Sword
Score 10 kills in one match with the Machinegun
Dive onto someone and kill them with the fall damage
Score 15 kills in one match as Dutch McClone
Score 15 kills in one match as Lord Edmund Gauntlet
Score 15 kills in one match as Mizu Ichiban
Score 15 kills in one match as Leona Wolf
Throw 5 items on the ground without hitting anything first
Dive through a window
Dive through 20 windows
Score 15 kills in one match as Lance Koboldski
Score 15 kills in one match as Mr. Shur Foo
Score 10 kills in one match with the SMG
Score 15 kills in one match as Hank Stream
Score 15 kills in one match as Hailey Skye
Score 15 kills in one match as Dr. Johnny Bionic
Score 15 kills in one match as Thelma Vaine