Here is the full list of all 42 Serious Sam Classics: Revolution achievements.
Find a secret Minelayer in The Second Encounter.
Find the XL4P Beam Gun outside of Bright Island.
Unfortunately, we don't have the right model for it, so this'll do.
Finish the Bright Island campaign in single player.
Find Bright Island's second secret level.
Deal a total of 1 million damage to enemies.
Find Bright Island's first secret level.
Finish The Second Encounter on single player.
Kill the boss and complete the Medieval episode.
Find a secret Plasmathrower in The Second Encounter.
Kill the boss and complete the Persia episode.
Kill the boss and complete the South America episode.
Obtain the Crystal Skull.
Finish The First Encounter on single player.
Find Sacred Yards.
Find the rising obelisk.
Defeat the giant lava golem.
All Elementals acquired.
Find Moon Mountains.
Kill the Highlander in Valley of the Kings.
Complete the first Egypt level.
Using explosive weapons, kill 10 people in a row.
Kill an enemy player while you're dead.
In a match of at least 6 players, run 2 kilometers without getting killed.
Kill 500 people.
Finish The First Encounter in cooperative.
Finish The Second Encounter in cooperative.
Find a secret bouncing rock.
Kill 100.001 enemies.
Mess with the gravity for a total of 60 minutes.
Kill 5000 marsh-hoppers with the chainsaw.
Complete a level in The Second Encounter using only the knife on at least normal difficulty.
Finish 2 levels in The Second Encounter using only the knife in at least normal difficulty.
Kill less than 8 enemies on the first level of The Second Encounter in at least normal difficulty.
Finish the first level of The Second Encounter taking no damage in at least normal difficulty.
Finish the first level of The Second Encounter without jumping in at least normal difficulty.
This achievement is impossible to get.
Kill 100 monsters with mines.
Kill 50 players with mines.
In singleplayer, beat the last level of The First Encounter or the last level in The Second Encounter in the "I am Ugh-Zan" difficulty.
Get the gold medal on the Christmas 2014 survival level.
Finish the Bright Island campaign in cooperative.