Halo 3

The best way to get this will be to turn on Campaign scoring and turn on a few skulls, I usually choose catch (easy since there's nothing but flood so its a freebie) , tough luck, thunderstorm, and if you know the map well enough you can use the fog skull too. Making it easier, you can do a normal run through. As long as you don't waste time and don't die to many times on that final warthog run, you should get this achievement.

EDIT: If you are playing with friends remember that you personally have to earn the 15k, not 15k between all of you!

This achievement is pretty simple, just turn on Campaign scoring free for all (it has to be you who gets the 15,000 points, so I'd suggest playing alone), and I personally suggest using Tough Luck, Catch, Fog, IWHBYD, Cowbell, and Grunt Birthday Party. (Obviously IWHBYD doesn't do anything except make the covenant say weird stuff :P) Catch and cowbell make using grenades much more useful, (cowbell makes explosions twice as powerful) tough luck doesn't make the enemy that much more difficult, and Grunt Birthday will make your grunts explode and maybe kill the grunts around them (cowbell helps here too). Obviously with fog, make sure to rely on your ears and sight and this achievement will be a breeze. :)

For this achievement you'll need to earn at least 15,000 scoring points on the final mission of the Campaign, Halo. This can be done solo or with a few buddies. (I got all mine solo and using the method below)

First you'll need to enable either Free-For-All or cooperative scoring from the campaign options. Then select the following for quickest results:

Normal Difficulty
Catch Skull - (1.00 multiplier)
Fog Skull - (1.00 Multiplier)
Famine Skull - (1.00 Multiplier)

So you'll be playing on Normal difficulty with enemies chucking nades alot, no Mini-Map, and ammo pick-ups are scarce. However, being on Normal difficulty will make this a cakewalk. And having these 3 skulls on will start your kill scoring at a x3 Multiplier, so as long as you finish the mission within a reasonable time, and die little-to-no times you'll have more than enough points to pop your achievement!