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