i'm not sure if this post exists somewhere else, i've looked and didn't find it so i gues it isn't. either way here are my tips for the meta-game (mods please move this to the right place if it isn't already)
1) it's easier to do the meta-game by yourself except in The Ark and The Covenant (which require 50 000 points as oppose to the 15 000 needed for the other levels)
2) Try to have Grunts Birthday, Cowbell and IWHBYD activated. It doesn't give you points but its entertaining.
3) Arrival (Sierra 117), Crow's Nest, Tsavo Highway, The Storm and The Ark are beaten the easiest when you play on Normal with Blackeye and Famine activated. Blackeye and Famine give a 3.5 multiplier before the Time Score. Using these 2 skulls will land you a 2.5 or 2.0 score time making the game that much easier.
4) Floodgate, Cortana and Halo should use the Catch skull since the Flood and Setinals don't throw grenades. An easy 1.5 multiplier with no extra challenge.
5) The Covenant is a level all its own for skulls. It should NOT have Blackeye unless you're amazingly good because of the area with the dogfight between the Banshees and Hornets.
If you run out of shield in the air, you're screwed. On top of that, the only place to get a shield in that section is the small island with 8 or so Brutes and if you kill all of them you lower your chances of winning. Have Famine active along with whatever skull works best with you. Remeber: this level needs 50 000 for the achievment so getting a minimum of 25 000 is a challenge, but getting a minimum of 25 000 AND finishing with a 2 or 2.5 Time Score is even harder.
6) In all the levels (and this works best for me but it might not work for you), fight your way through until you reach about half of the required score (25 000 for Covenant and Ark, 7 500 for the rest) and once you get it, run your way through the rest of the level focusing mainly on the Time Score.
Obviously you'll want to kill everything that may stop you, but don't dawdle on the small stuff.
The 3 minutes it takes to kill 10 Brutes can make or break the total score.
