Halo 2 Hints
If You complete the following tasks Yoiu'll earn the desired achievement. Keep in mind these achievements work only on the PC version.
Warrior (100 points) - Complete the game on Normal difficulty.
Legend (50 points) - Complete the game on Legendary difficulty.
Hero (50 points) - Complete the game on Heroic difficulty.
Demon (25 points) - Complete any level without dying once and the tutorial level does not count.
Violent Cartographer (25 points) - Play every default map and stay in the game until it's over.
Rainman (25 points) - Play every game variant, with at least 3 other players, until the end of the game.
Double Kill (20 points) - Kill any 2 opponents within 4 seconds.
Air Traffic Control (20 points) - Blow up the Banshee in flight while manned, with grenades or the rocket launcher.
Go Ape Shiv (20 points) - Kill an enraged, berserk Brute by melee.
Hired Gun (20 points) - If you know someone in the game who has completed the game on Legendary, then kill him.
Killing Spree (20 points) - Kill 5 opponents in a row in the same game, without dying.
Killtacular (20 points) - Kill 4 opponents.
King of the Scrab (20 points) - Aquire the Scrab gun on the Metropolis level.
Ninja (20 points) - In a non-team game, kill 5 people by melee, from behind.
Running Riot (20 points) - 10 opponents, same condition.
Skewer Stopper (20 points) - Kill the sword carrier, after he/she has killed 5 or more opponents in a row with the sword.
Slient but Deadly (20 points) - Kill 7 enemies in a row without being spotted.
Triple Kill (20 points) - 3 opponents.
Assassin (10 points) - Melee an opponent from behind.
Bonecracker (10 points) - Hit and kill an opponent from behind.
Decorated Soldier (10 points) - Get awarded at least 8 different medals in one non-team game.
Roadkill (10 points) - Run over, hit and kill an opponent with a vehicle.
Carjacking (5 points) - Steal an occupied vehicle from an opponent and take it over.
Counterpoint (5 points) - Kill the sword carrier.
Stick It (5 points) - Stick an opponent (MP) - with a plasma grenade and blow them up.
(Campaign level goes here) (30 points each) - Complete each level (seperate achievements) on Normal, Heroic, or Legendary. There are 13 levels (not counting tutorial) for a total of 390 points.
How to obtain the "Silent But Deadly" achievement effortlessly
You cann find any level with a grunt or two sleeping, or even an enemy around that can be easily assassinated from behind without alerting anyone else around (This enemy should preferably be right after a checkpoint). After you've done this, assassinate them. Be sure to hit the enemy with the reticule not directly on the enemy, as there is no chance for the enemy to fire a few shots off while dying and alerting others nearby. Once you've successfully killed the enemy/enemies, reload to the last checkpoint. From there, just find those enemies again and kill them as before. Keep doing this until you get the achievement, but without getting caught in between loads. It seems that the game has a glitch, which doesn't reset the "kills without being caught" meter at every reload. May as well exploit it.