Halo: Reach

I'm currently still working on this and on mission 5. There's no "specific way" to get this achievement, but I will give you some tips.

** = Strongly Recommended
* = Recommended

**If your getting this achievement in Co-op like me, then you should use this strategy for EVERY LEVEL!
It doesn't matter if your doing this with 2, 3, or 4 people.
Have 1 person stay back at a safe spot while the other person(s) go up and kill enemies, the first person to die stays back and the other person goes. This helps very much!

**When fighting lots of enemy's in a small area try to only kill a couple and find a route to run through and trigger a checkpoint. The only time you can't do this is if it says you have to "Defend" something. (When you "defend" stuff you just have to kill all the people)

*If you find a vehicle and are headed to an area filled with lots of enemy's then save the vehicle until you've killed about half of them then use the vehicle so you don't go out and get killed in one shot by a Wraith.

*Try not to use "good" weapons right away (Sniper, Rocket Launcher, Shotgun, etc.) This helps because if you save it and don't run in and end up using it on some Grunts your gonna regret not saving it for an Elite/Hunter.

Those are some basic strategy's that I use and it's making Campaign go alot faster for Local Co-op. But these still work on Xbox Live Co-op! Let me know if you have any more tips I could add.

Honestly, I found the best way to get this achievement is to simply get as many of your friends together as possible to run through the game. If you can pull 4 people together, the game will STILL be hard, but really possible. Just keep in mind that running past problem areas will work, so long as you can keep moving and/or not get spotted by the enemy.

A great strategy when using co-op is to have at least one person stick back as far as possible. This way, when someone dies (and they will), there will always be a good spawn point for the dead man to return to the fight. It won't work in all areas, but it's a solid rule of thumb for most of the game.
Focusing your fire on the same targets will also help, especially if living marines are trying their best to stay alive. This helps keep yourselves alive, as well as those marines so they can soak up more damage for you in more important areas of the game.

Either way, if you can persevere (it took my friends and I a loooooong night to get it done), it shouldn't be too hard to finish it.
Just expect to die.
A lot.

I've created a full walkthrough video (all missions in one convenient video) for the campaign on Solo Legendary. You'll unlock this achievement along with all the other difficulty achievements, as they stack, if you run through solo Legendary following the video (which will also point out any and all other achievements you can get along the way).