Accomplish the indicated achievement to get the corresponding number of 
Gamerscore points:

Piece of Cake (25 points): Complete "L.A. Meltdown" on any difficulty or in co-op
Let's Rock (25 points): Complete "Lunar Apocalypse" on any difficulty or in co-op
Come Get Some (25 points): Complete "Shrapnel City" on any difficulty or in co-op
Game Over (25 points): Complete "The Birth" on any difficulty or in co-op
Duke's Mighty Foot (10 points): Step on 40 enemies after using the Shrinker on them
Looks Like Cleanup on Aisle 4 (10 points): Step in a pile of excrement. 
Shake It Baby (10 points): Tip an exotic dancer. 
Oomph, Uugh, Where is it? (10 points): Find 70 secret areas. 
Alien Maggots are Gonna Pay (10 points): Finish the "Hollywood Holocaust" level
Gonna Rip Em' a New One (10 points): Get 100 total kills in ranked Dukematch games
See You in Hell (15 points): Get 250 total kills in ranked Dukematch games
Hail to the King, Baby (25 points): Earn respect and get 500 total kills