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