Here is the full list of all 63 Gunslugs achievements.
Kill 5 skulls in one mission
Complete mission 5.2
Complete mission 5.1
Complete mission 4.3
Complete mission 4.2
Kill a flying fire skull
Get your train ticket and ride.. or run
Score 2500 points
Find the sketch world!
Rescue Sarge
Complete mission 4.1
Complete mission 3.3
Drive over 10 dudes with a tank
Drive over 5 dudes with a tank
Kill a chicken, and spread some eggs!
Rescue B.A. Baracuda
Destroy four mechballs
Collect 175 coins in one game
Locate the arcade !
Chain explosion of 3
Score 2000 points
Grab energy drink
Complete mission 3.2
Kill 50 guys in one mission
Blow up 4 dudes at once
Complete mission 3.1
Chain explosion of 2
Collect 150 coins
Score 1000 points
Rescue Willis KiYay
Complete mission 2.3
Kill 10 guys with grenades
Take a jetpack ride
Rescue Sonya
Kill 20 guys
Find "a" princess
Complete mission 2.2
Rescue Sly Rocko
Drive a tank
Complete mission 2.1
Collect 125 coins in one game
Collect 100 coins
Score 500 points
Kill 5 guys from cover
Complete mission 1.3
Change the tune using a jukebox
Air kill a dude
Blow up 2 dudes at once
Find the voodoo priest
Kill 10 guys in a mission
Complete mission 1.2
Make a chopper crash
Complete mission 1.1
Collect 75 coins
Grab a new weapon
Complete bootcamp
Deactivate your first beacon
Take cover rookie!
Blowup a death worm
Destroy 5 hell tanks in one game
Complete mission 5.3
Complete Hell with Sarge
Complete Hell with BA