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.
Find 70 secret areas
Complete "Duke Caribbean: Life's A Beach" on any difficulty
Complete "Lunar Apocalypse" on any difficulty
Complete "Duke: Nuclear Winter" on any difficulty
Complete "Duke It Out In D.C." on any difficulty
Complete "The Birth" on any difficulty
Complete "Shrapnel City" on any difficulty
Step in a pile of excrement
Kill 1000 enemies
Find Luke Skywalker on "Lunar Reactor" level
Find the Doomguy on "Death Row" level
Kill every enemy on a level
Complete "L.A. Meltdown" on any difficulty
Tip an exotic dancer
Step on 40 enemies after using the Shrinker on them
Finish the "Hollywood Holocaust" level in under 3 minutes
Use a cheat code
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)