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