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 "The Birth" on any difficulty
Complete "Shrapnel City" on any difficulty
Complete "Lunar Apocalypse" on any difficulty
Find Luke Skywalker on "Lunar Reactor" level
Step in a pile of excrement
Complete "L.A. Meltdown" on any difficulty
Find the Doomguy on "Death Row" level
Earn respect and get 500 total kills in Dukematch games (Online)
Get 250 total kills in Dukematch games (Online)
Get 100 total kills in Dukematch games (Online)
Tip an exotic dancer
Finish the "Hollywood Holocaust" level in under 3 minutes
Complete "Duke Caribbean: Life's A Beach" on any difficulty
Complete "Duke: Nuclear Winter" on any difficulty
Step on 40 enemies after using the Shrinker on them
Kill 1000 enemies
Use a cheat code
Complete "Duke It Out In D.C." on any difficulty
Kill every enemy on a level