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