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