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