Here is the full list of all 12 Serious Sam HD: The Second Encounter achievements worth 200 gamerscore.
Complete any level in single player.
Complete the game.
Complete the game in single player on serious difficulty.
Find at least 80 secrets in single player.
Complete The Grand Cathedral level on serious difficulty without dying or loading.
Complete the entire Coin-op coooperative game on normal or higher difficulty.
Win a CTF match with at least 2 players on each team.
Win one My Burden match with at least 3 players.
Win a Beast Hunt match on all game levels.
Earn at least a bronze medal on any level in single player Survival.
Win 100 deathmatch games.
Accumulate a total of 100,001 enemy kills and frags combined.