Here is the full list of all 42 Serious Sam Classics: Revolution achievements.
Finish the Bright Island campaign in single player.
Finish the Bright Island campaign in cooperative.
Find Bright Island's second secret level.
Find Bright Island's first secret level.
Kill 100.001 enemies.
In a match of at least 6 players, run 2 kilometers without getting killed.
Kill less than 8 enemies on the first level of The Second Encounter in at least normal difficulty.
Finish 2 levels in The Second Encounter using only the knife in at least normal difficulty.
Finish The Second Encounter in cooperative.
Mess with the gravity for a total of 60 minutes.
Kill 500 people.
Kill 50 players with mines.
Using explosive weapons, kill 10 people in a row.
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.
Finish the first level of The Second Encounter without jumping in at least normal difficulty.
Finish The First Encounter in cooperative.
Get the gold medal on the Christmas 2014 survival level.
Kill 5000 marsh-hoppers with the chainsaw.
Kill 100 monsters 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.
This achievement is impossible to get.
Finish The Second Encounter on single player.
Kill the boss and complete the Medieval episode.
Find a secret Minelayer in The Second Encounter.
Find a secret Plasmathrower in The Second Encounter.
Kill an enemy player while you're dead.
Kill the boss and complete the Persia episode.
Deal a total of 1 million damage to enemies.
Kill the boss and complete the South America episode.
Obtain the Crystal Skull.
Find a secret bouncing rock.
Unfortunately, we don't have the right model for it, so this'll do.
Find the XL4P Beam Gun outside of Bright Island.
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.