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