Here is the full list of all 26 Serious Sam VR: The First Encounter achievements.
Trigger Ugh-Zan's regeneration.
Kill 100 Enemies with the Cannon.
Kill 300 Enemies with the Minigun.
Kill 150 Enemies with the Rocket Launcher.
Find at least 10 secrets in single player.
Complete any level in co-op.
Kill 20 enemies with the Minigun without releasing the trigger.
Obtain the Water Element.
Kill 200 Enemies with the Thompson Submachine gun.
Kill 250 Enemies with a shotgun.
Obtain the Fire Element.
Obtain the Earth Element.
Obtain the Air Element.
Complete any level in single player.
Pull out a single Colt and immediately deliver a killing shot.
Defeat the Lava Golem boss.
Pierce at least 10 enemies with a single cannonball.
Leap into the unknown on Luxor.
Complete the game.
Complete the game in single player by beating the estimated time on each level.
Complete the Metropolis level in single player on unmodified serious difficulty.
Complete the game in single player on unmodified serious difficulty.
Complete a level in co-op with blood option set to hippy.
Complete the game in co-op on unmodified serious difficulty.
Win a versus match.
Frag a total of 100 players.