Successfully complete the indicated task to unlock the corresponding achievement.

Achievement	How to unlock
Are You Serious?	Complete the game in single player on unmodified serious difficulty.
Bird Hunter	Kill 20 Harpies with the Sniper rifle.
Burn Baby, Burn	Burn 100 enemies to death.
Co-op Beginner	Complete any level in co-op.
Co-op Survivor	Earn a gold medal on any level in cooperative Survival.
Crunchy	Kill 100 Kleers with the Cannon.
Fragger	Frag a total of 100 players.
Look Ma, I won!	Win a versus match.
Look Ma, I'm a Lumberjack!	(secret)
Look, it's a secret!	Find at least 10 secrets in single player.
Make it count	Kill at least 30 enemies with a single Serious Bomb.
Medieval Master	Complete the Medieval episode in single player or cooperative.
Mine's bigger than yours	Destroy an enemy cannon with your own cannon.
Nice Try	(secret)
Persia Master	Complete the Persia episode in single player or cooperative.
Rrrrrrrrrr	Kill 10 enemies with the Chainsaw without releasing the trigger.
Serious Beginner	Complete any level in single player.
Serious Damage	Kill 100 enemies with Serious Damage active.
Serious Run	Complete the game in single player by beating the estimated time on each level.
Serious Sam	Complete the game.
South America Master	Complete the South America episode in single player or cooperative.
Space Marine	Kill 200 enemies with the Laser gun.
Survivor	Earn a gold medal on any level in single player Survival.
The Beast	Finish the Legend of the Beast campaign in single player or cooperative.
The Perfect Storm	(secret)
Treasure Diving	Find a secret under water.
Trick or Treat	Kill 50 Cucurbitos.
Wall of Bullets	Kill 20 enemies with the Minigun without releasing the trigger.