Here is the full list of all 20 Duke Nukem 3D: Megaton Edition achievements. It takes around 10-12 hours to unlock all of the achievements on Windows.
Complete "L.A. Meltdown" on any difficulty
Complete "Lunar Apocalypse" on any difficulty
Complete "Shrapnel City" on any difficulty
Complete "The Birth" on any difficulty
Step on 40 enemies after using the Shrinker on them
Step in a pile of excrement
Find 70 secret areas
Finish the "Hollywood Holocaust" level in under 3 minutes
Complete "Duke It Out In D.C." on any difficulty
Complete "Duke: Nuclear Winter" on any difficulty
Complete "Duke Caribbean: Life's A Beach" on any difficulty
Kill 1000 enemies
Kill every enemy on a level
Find the Doomguy on "Death Row" level
Find Luke Skywalker on "Lunar Reactor" level
Get 100 total kills in Dukematch games (Online)
Get 250 total kills in Dukematch games (Online)
Earn respect and get 500 total kills in Dukematch games (Online)
Use a cheat code
Tip an exotic dancer