Here is the full list of all 20 One Gun 2: Stickman achievements.
Kill hell boss
Kill dungeon boss
Kill city boss
Complete all points in the hell at level 3
Complete all points in the dungeon at level 3
Complete all points in the city at level 3
Kill hell boss level 2
Kill hell boss level 3
Kill dungeon boss level 2
Kill dungeon boss level 3
Kill city boss level 2
Kill city boss level 3
Kill ocean boss
Kill ocean boss level 2
Kill ocean boss level 3
Complete all points in the ocean at level 3
Kill space boss
Kill space boss level 2
Kill space boss level 3
Complete all points in the space at level 3