Here is the full list of all 12 Serious Sam HD: The First Encounter achievements worth 200 gamerscore.
Complete any level in single player.
Complete the game.
Complete the game in single player on serious difficulty.
Complete the game in single player without firing a single bullet or shell.
Complete the game in single player by beating estimated time on each level.
Complete Metropolis level in single player on serious difficulty without loading.
Pierce at least 10 enemies with a single cannonball.
Kill all enemies in Karnak in single player.
Find at least 50 secrets in single player.
Complete any level in co-op.
Complete The Great Pyramid in co-op with blood option set to hippie.
Complete a co-op game on serious difficulty and extra enemy strength on 400%.