Here is the full list of all 30 Serious Sam Double D XXL achievements. It takes around 6-8 hours to unlock all of the achievements on Windows.
Find ALL Secrets
Find or reach 100% levels, challenges, secrets, guns, and connectors
Unlock ALL golden guns
Clear ALL Challenge levels
Find ALL guns
Clear 8 Challenge levels
Kill a Giant Flea in Fireball Fury
Unlock 2 golden guns
Kill 10 Gnaar on ceiling spikes in Amethystine Depths
Help Templeton unlock his secret room in Amethystine Depths
Find all 4 of any gun type
Clear 2 Challenge levels
Beat the game on any difficulty
Replay any previous level (Choose Level menu)
Beat any level without getting hurt
Find 30 Secrets
Find the Secret Red Light District
Kill over 50 Gnaar in a single level
Find 16 guns
Complete a level from the start at over 100% gameplay speed
Kill a Red Biomechanoid with only chainsaws
Rescue 3 dinosaurs by destroying their helmets
Find the Monument to Parallax
Create your first 6 gun stack
Eat one Boston Creme or Cannoli
Find the Jump Pad
Beat any level in under 2 minutes in its entirety
Find Mental's Secret Closet
Find 10 Secrets
Beat any level without dying