Halo 3

This is a pretty easy achievement. For starters, play only by yourself. Play on the Normal difficulty, and put these skulls on: Mythic, Thunderstorm, Catch, Fog. These practically make sure that even if you don't reach 15,000 in-game, the multipliers ensure you get it. Make sure campaign scoring is on.

Now, once you're playing. You want to play in a certain way. If you're driving a warthog, make sure a marine is in your turret. The points for any kill your Marine gets goes to you. Also, try to make specialty kills. These are needler kills, sticks, headshots, anything that's above your average kill. These give you extra points. Go for vehicle kills. They give you a lot of points. Try not to die a lot.

Follow these points and you'll get the achievement guaranteed.

I found that playing on Normal, with a second controller as a re-spawner (they will continue to advance as you reach checkpoints), and putting on Tough Luck, Famine, Fog, and Thunderstorm gave a beneficial 6.5x multiplier. This method can be used for practically any level of the game.

Now, level specifics. Drive the warthogs for any possible part of the level you can and don't waste time. Using these tactics I easily surpassed the required points with 50,000 with only dying twice or three times.Good luck.

An important thing to note is that your score during the game includes your skull multipliers. So the only thing you will get at the end of the game in addition is the time bonus.

So if you have lost a great deal of your time bonus, then don't continue to play the level thinking there is more bonuses at the end, because there is not.

It is your style of play that determines the skulls you activate. My best advice is to play on Normal. The reasoning behind this is that where Legendary will net you points, you have a better chance of hitting the time bonus on Normal, and you can have more skulls on at once and it still will not be a big deal. Thus more bonus points.

Start with three or four skulls on, and see how you do? If it is too hard, then start over and remove a skull.

One the other hand, if you can net the required amount of points, then the time means nothing. On Tsavo Highway for instance, I hit the 15,000 points in game. So I just jumped in a ghost and skiddaddled to the end (running through the rest of the action), and got the achievement.

And don't be afraid to die a few times, on Normal you don't lose that many points. Of course on Normal you shouldn't die all that much either. Time is the biggest bonus, try for that before you waste time lining up every headshot and multi-kill.

Good Luck!

For this achievement you'll need to earn at least 15,000 scoring points on the third mission of the Campaign, Tsavo Highway. 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!