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