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