Listed below are various strategies for multiplayer (or skirmish) games. Note that this is an incomplete page, but there will always be people adding on to this page.
The Rush strategy is very basic, but can also be very crippling. This strategy involves building units very quickly, and attacking before the enemy has a chance to react. However, if you fail, this will leave you wide open to a counterattack, which will be far more crippling to you. Since there are three UNSC factions, there are different units that are open to them at the beginning. Captain Cutter uses Elephants, Forge with Cyclops, and Anders with Gremlins. We will start with Captain Cutter.
Captain Cutter has a distinct advantage over the others. He is able to establish mobile barracks anywhere on he map with Elephants. This vehicle, if used properly, can be an excellent seige weapon. Or for this strategy, a rush tool.
First, build an Elephant. You only need one, and making another takes away resources that will be used to train units.
Next, send it right outside an enemy position. If you judged your distance correctly, the enemy will not know you are there.
Then, start training Marines. Other units aren't an option because (A) Only Marines have an ability that does damage, and (B) There are more soldiers per squad, whereas Flamethrowers come in two's and Spartans are singular.
Now your starting unit (Warthog) comes into play. Use it to distract the enemy warthog (or ghost/chopper) by ramming it. It is a significant threat to your infantry with its "Ram" ability (or ghost cannons).
Finally, Begin your attack. Skip any structures he's built and go straight for the main structure. Open with grenades, then fire. If the enemy has trained units, then send your Elephant in to destroy them. DO NOT let the marines stop attacking the structure. Any time wasted is time the enemy has to counter your assault.
If the enemy has built a Temple (has a hero on the battlefield) then I recommend stopping him with your Elephant. If he is too strong, then the Elephant will have at least drawn the attack away from your marines. By now you should have done significant damage to the enemy, if not destroyed his base. If this rush fails, there is a very high chance he will strike back immediately.
With Forge, things are different. He has no Elephants, so he can't deploy troops close to the enemy. However, he can build Heavy Supply Pads right away, which can fuel a rush much better than with Captain Cutter.
First, build one Heavy Supply Pad and one Barracks.
Next, start training Marines. Marines are the preferred unit for reasons described above. It is also optional to build a Cyclops. They are excellent building killers, but costly.
Then you begin your assault. Try and attack from different directions if possible. Also, use your Warthog to scout ahead. Or collecting supply crates is also acceptable. Using the credits from your Supply Pad and crates can expand your army to a much greater mass than with Cutter.
You can use the warthog to scout, then use a Pelican to drop your assault team at their base. Have the Warthog distract the enemy scout unit (Warthog/Ghost/Chopper)from your infantry. Once your infantry have touched down, immediately assault with grenades. Then, go back and train more Marines, and drop them off with Pelicans at the enemy base.
If the enemy has a hero, and you have a Cyclops, let some Marines distract him. The Cyclops should compensate. Remember to check all unit abilities for their recharge time. Immediately throw more grenades when you can.
This strategy has a higher success rate, though takes a bit longer to accomplish, giving the enemy time to react.
Anders uses Gremlins as a special units. These fire lasers roughly the equivalent of a Spartan Laser, and have an EMP burst that stuns enemy vehicles. Therefore this makes the Gremlin an excellent rush unit. Also remember that Anders has a special "Cryo Bomb" attack that can stun enemy units, and halt enemy construction. Against a Covenant player, this can be crippling, as he will most likely construct a Temple first.
Gremlins are a costly unit, so collect crates with your Warthog. Do not bother with Supply Pads, they are too expensive, and you need as many Gremlins as you can get.
Once you have as many as you can, go assault the enemy. Target the main structure, avoid any others he may have built.
If he is in construction of a Temple, target it and destroy it while in its early stages of construction. If you target any structures he is building (that pose a threat, i.e. Hall/Barracks, Temple, etc...) and destroy them before they are built, you may be able to suppress the enemy long enough to destroy him.
Other forms of Rush strategies include an Air Rush. You build Supply Pads and a Reactor. With enough credits, you can create a small squadron of Hornets. The enemy will most likely not see this coming, but it will also take a good ten minutes at the least.
The Covenant have a different take on Rushes. Each faction builds a cost-effective infantry. The Arbiter trains Suicide Grunts, the Brute Chieftain constructs Brutes, and Regret creates Honor Guards. We will start with the Arbiter.
The Arbiter uses a very unique variant of the Grunt. These Grunts can detonate themselves with area affect damage. However, the key is to NOT use this ability until the unit is close to death. Also, including the Arbiter himself can prove very useful. He can eliminate any infantry the enemy can pull out, and can prove to be a match for any hero. However, this will limit the amount of units you can send in with him.
Also note that these Grunts do about as much damage as regular Grunts, so they won't do much. So you can also create a Hall and train Grunt Squads (or hunters, with limited numbers). These have a recharging "Plasma Grenade" ability that, when used in mass attacks, can be devestating.
The Brute Chieftain produces Brute infantry units. Each squad comes with two Brutes, but they are stronger than other infantry. Their Brute Shot cannons can do massive damage to infantry and, if used properly, structures. Though they are small in numbers, they make up for it in firepower, and that is just what you need to win the game. The Chieftain is also powerful against infantry and light vehicles. He can defend the Brutes from attacks while they dish out damage. The Chieftain is not suited for structures, and you won't have enough credits to make full use of his "Vortex" ability.
The Prophet of Regret trains a very interesting unit. The Elite Honor Guard uses Energy Swords similar to the Arbiter, and therefore lack a ranged attack. So it is not recommended you use these units for a Rush. However, you can use Hunter Pairs. Though you should only be able to build four pairs, they can do great damage to infantry, vehicles and structures. With them, target any structures that are being built. This costs the enemy time and credits, and makes your assault much more successful. However, Hunters can be overwhelmed if you aren't careful.