The lights are on
So, yes, there are a number of these guides already in existence, but they don't seem to go into a lot of detail...plus they're just not how my friends and I did it. This is the method (somewhat taken from internet tips and somewhat on the fly) that got my friends and I into set 5 with about 15 lives to spare.
Head to Lost Platoon. Some things you might want from the very beginning:
1. A designated Spartan Laser player (for taking out those annoying Wraiths quickly) and/or a team of two assigned to highjacking the wraiths (Halopedia has a good explanation on how to do it...and yes, it's a glitch exploit, but it will save you a lot of headaches in the early goings, especially if you've got a squad member who's good with the mortar launcher).
2. A team of two to drive the Warthog and man the turret. A good driver can get a decent amount of splatters (even with tough luck turned on) while the gunner cleans up the leftovers.
3. A pistol headshot specialist for the bonus rounds. In the right hands, the pistol can be a headshot machine, great for taking out large clusters of Grunts quickly for huge points.
4. Possibly a sniper. Until Tilt turns on, Sniper support can be great for taking out Brutes.
General strategy pre-Tilt:
Avoid direct on-foot confrontations pretty much at all times. There should ALWAYS be one player holding on to the Spartan Laser (again, thanks to Halopedia for the map of weapon drops) to pick off Wraiths quickly from the center structure or a team of two in a highjacked wraith. Also, if you're driving anything - be it the wraith, warthog, or a highjacked chopper - keep moving at all times. Learn where the obstacles are so you don't accidentally ram them and get stuck. This is doubly important to remember whenever Catch is activated, as even a moments pause can quickly turn fatal. if you are on foot, whatever weapon you choose, make sure you've got at least one that will help you adequately deal with drones (for me that means SMG pre-tilt, plasma rifle post-tilt), as they have the highest potential to turn your game sour in the blink of an eye. Again, remember the pistol can be grunt-headshot machine in the right hands.
Remember, KEEP MOVING. If catch is on, never stop driving, or if you're on foot, remember - jumping is your friend. The good news about catch is you'll usually never be wanting for grenades of your own, and with the slow throwing animations your enemies have, you can, if you're a decent throw, stick an enemy or two of your own.
During bonus rounds, if you're not so good with a pistol, a well-placed grenade is also a good choice, but I prefer either a fuel rod gun or gravity hammer myself, which you'll almost always have access to in the last wave of the last round, when brutes come en masse. You might die quickly if you rush head-on into a group of grenade-crazy grunts, but odds are you'll be able to get a good swing or two in first, usually resulting in a sizable multi-kill medal (my personal record is eight in one swing).
Once tilt is activated, your strategy will change drastically. Improved bullet resistance means no SMGs, no Snipers, and no Warthog (but a pistol in the bonus rounds is still a good idea if you're good enough). But here's where the strategy gets a littler simpler.
Step one, grab a plasma pistol or two. Step two, overcharge a Chopper and hijack. Step three, start doing laps. Rinse and repeat.
Seriously, if everyone finds a chopper and gets their NASCAR on, Tilt shouldn't prove as difficult as you might fear. Make sure you agree on a direction in which to travel, though, as accidental betrayal-splatters can be costly. Keep your finger on the trigger, and go for splatters whenever you can - a Chopper boost is GREAT for quick kills on tougher enemies. It won't be a quick process, but you wouldn't be going for this achievement if you weren't in it for the long haul, right?
Make sure you keep a spartan laser handy for the wraiths, and I like to keep a gravity hammer handy for dealing with Black Eye - if you're quick, you can rush into a group of enemies to make them dive, ditch the chopper, thwack 'em on the head, and jump back in before they know what hit them - or just keep your eyes out for lone enemies to punish. Always be sure to keep communication going between your squad when you ditch your vehicle, though, so as to avoid any accidental betrayals. And even if a melee opportunity fails to present itself, the extra damage protection a vehicle provides can keep you going for much longer than you'd otherwise be able to.
That's about all I can think of. Again, this got me and three friends into the fifth set with many, many lives to spare...and remember, this is also good if you've yet to get the 200k points achievement, as you should be well past the million-point mark by the time set 5 rolls around.