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