Halo 3

This achievement should be a fairly easy one to get. Turn on Campaign scoring then turn on a few skulls, I usually use Tough Luck, Famine (since you spend a bit of the mission in vehicles), Thunderstorm, and the "grenade happy skull" Catch. Now, simply do a normal run through of the mission. All the vehicles you kill along the way (including the scarab) should most likely get you this achievement. Just don't waste too much time and this is one is in the bag.

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

Can easily be obtained (like all of them) by running through the level(s) getting headshots and multi-kill medals. Getting chain kills will usually be the secret to obtaining these kind of achievements. Avoid dying, and if possible, try to play with a really good friend (if your not up to par). Dying constantly will drop your score drastically.

For this achievement you'll need to earn at least 15,000 scoring points on the fourth mission of the Campaign, The Storm. 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!