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