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.
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.
Kill 100.001 enemies.
Kill 500 people.
Kill 50 players with mines.
Kill 100 monsters 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.
Deal a total of 1 million damage to enemies.
Kill an enemy player while you're dead.
Find Moon Mountains.
Finish The Second Encounter on single player.
Finish The Second Encounter in cooperative.
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 the boss and complete the Persia episode.
Finish 2 levels in The Second Encounter using only the knife in at least normal difficulty.
Kill the boss and complete the South America episode.
Mess with the gravity for a total of 60 minutes.
Kill 5000 marsh-hoppers with the chainsaw.
Obtain the Crystal Skull.
Finish the first level of The Second Encounter without jumping in at least normal difficulty.
Finish the first level of The Second Encounter taking no damage in at least normal difficulty.
Kill less than 8 enemies on the first level of The Second Encounter in at least normal difficulty.
This achievement is impossible to get.
Complete a level in The Second Encounter using only the knife on at least normal difficulty.
Get the gold medal on the Christmas 2014 survival level.
Find a secret bouncing rock.
Finish The First Encounter in cooperative.
Finish The First Encounter on single player.
Find Sacred Yards.
Find the rising obelisk.
Defeat the giant lava golem.
Find the XL4P Beam Gun outside of Bright Island.
Unfortunately, we don't have the right model for it, so this'll do.
All Elementals acquired.
Kill the Highlander in Valley of the Kings.
Complete the first Egypt level.