Here is the full list of all 19 HUNTDOWN achievements.
Get 100% completion in Hard Mode
Find at least 5 secret locations
Get 100% completion in Normal Mode
Destroy 5 cars at the Overseer's Fume Pit without dying
Make Suki the Sniper kill herself with her own missile
Kill 10 enemies at close range with a Sawed-off on one level
Kill a dog by making it fetch a grenade
Press the retry button after completing a level
Trash a guitar on Sid Handsome
Kill 10 enemies with a Baseball Bat on one level
Complete a level without picking up any weapons
Kill a bounty while having one health point left
Defeat the Shogun
Restart more than 5 times during a fight with a bounty
Watch all of Wolfmother's briefs without pressing skip
Get hit by Nadja Drago's hockey stick and land in the goal
Kick and shoot 10 enemies with a Shotgun before they hit the ground on one level
Get kicked out by Tony 5 times at the same checkpoint
Get 100% completion in Badass Mode