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