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