Completing all the scores on any Halo campaign can be a pain. For the most part the guides for each level are spot on, but I'm surprised that nobody has suggest a key thing that helped me complete a lot of these..... Playing co-op.
If you are struggling to complete a mission and have a friend willing to help, it can make the process so much easier. Although I didn't complete all the missions with this method, it helps on some of the longer missions filled with more enemies.
The basics: Whoever is going for the high score is obviously killing everything. Your co-op partner is taking a backseat, assisting when needed. Once the score is obtained and you finish the level, trade places. I also suggest having the person going for the score to host, as the non-host may run into lag issues that may effect them. There are some levels that have enough enemies you can complete the score at the same time.
Your co-op partner can help you out in many ways.
1) Iron skull: A lot of guides I've read say that if you die, you can save and quit, then load the game back where you were. I did not have succes with this. Once I completed the level with enough points, I could never get the achievement to pop. Not saying it doesn't work, but it didn't work for me and turned into a huge time waster. In co-op you only get reset to your last checkpoint. Still try not to die as you loose yor kill streak bonus. I haven't confirmed this but if your partner dies, I don't believe you both loose your kill steak bonus.
2) An Extra Pair of Eyes: With Fog and other various skulls locating enemies to either move forward in a mission or especially eliminating a sniper can be a pain. Your partner can help call out what is happening on the battlefield when you are focused on killing. (flanking enemies, grenades incoming, weapon pick-ups, etc).
3) Crowd Control: Some areas have a lot of enemies in small areas (especially Flood) crowd control with both of you focusing fire can make a lot of these areas easier.
4) Pack Mule: Load up your co-op partner with weapons you might need later on. With Famine on, maintaining your ammo reserves can get sketchy in some areas or you may need a certain type of weapon later on. Having your co-op partner bring it with them and dropping it for you helps a lot.
5) Plasma Pistol: Elites and sometimes Jackals (especially with Thunderstorm on) can be difficult to get through. Your co-op partner can help drop shields with a charged plasma pistol while you pick off head shots.
6) Driver: I didn't run into this a whole lot in Halo 2, but more in Halo: CE. The AI can be terrible at driving/shooting in warthogs. With a co-op partner you always have a hopefully intelligent person to help position you to rack up kills and keep you out of harms way.
7) Diversions: With Anger, Catch, and That's Just Wrong on enemies are always focused on the player. It helps when you are trying to recover your shields to have someone else back you up.
8) Game Triggers: There are a few areas that trigger events once you pass through them or you need to finish a level to trigger a cut scene quickly as your points are decreasing. You can work together with your co-op partner to move through levels more efficiently and trigger enemy spawns so both of you are not taking heavy fire. Also the person going for the score can rack up kills while your co-op partner moves to the next area and then has you spawn on them. Again I noticed this more for Halo: CE, but there were some areas that this is useful in Halo 2.
9) Eliminating Friendlies: Some levels have Friendlies that "assist" you in your fight. I mostly found they got me killed by grenades and stole my kills. Your co-op partner can sabotage them with a plasma pistol or kill the out right. Even though your score decreases right there as it is cumulative, the negative points will only come off that individuals base score and not the person going for the high score, so it has little to no impact once the level is finished.
Again these are just some pointers I founf that helped me on some difficult levels. Going for scores is a big time drain, playing levels over and over. Playing with a friend makes it a little easier, adds some fun, and makes the whole experience a little less frustrating. Using this method and planning ahead what skulls and difficulty to play on, made getting the scores a lot easier.
So far I used this method on troubling levels on Halo: CE and Halo 2 and plan on using it for the other campaigns.
Leave any comments or suggestions!