Here is the full list of all 30 Serious Sam: Double D XXL achievements worth 400 gamerscore.
Buy at least one of every upgrade
Find All Secrets
Find all guns
Clear all Challenge levels
Buy 16 different upgrades
Beat any level without getting hurt
Trip a Therizinosaurus with the Hot Butter Spreader
Buy 4 different upgrades for one gun type
Clear 8 Challenge levels
Beat the campaign on any difficulty
Kill a Giant Flea in Fireball Fury
Find the Secret Red Light District
Defeat the boss of Prehistoric Nova Scotia
Find all 4 of any gun type
Convert 3 dinosaurs to your side by destroying their helmets
Find 16 guns
Find the Secret Monument to Parallax
Help Templeton unlock his secret room in Amethystine Depths
Beat a challenge in coop
Replay any previous level with new inventory (Choose Level menu)
Beat a level in coop
Complete a level from the start at over 100% gameplay speed in one sitting.
Revert at least one gun to get some money back
Clear 2 Challenge levels
Beat any level without dying
Defeat the boss of Ancient Egypt
Find the Jump Pad
Create your first 6 gun stack
Kill over 50 Gnaar in a single level
Die in the spikes in the tutorial area of level 1