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