This map is most likely part of the Heretic Base and Gas Mine. It consists of four main parts. The first section is the long corridor in the middle of the map. Here you can find an Overshield on the farthest way from the gravity lift (aka: Really BIG Man Cannon). This long corridor acts a large gorge between the red and blue sides of the map. Both sides along with the corridor in the middle lead to the fourth part of the map that is the ramp area. Here you can find a Shotgun at the bottom of the ramp and a Beam Rifle at the top of the lift that propels you from the lower corridor to the upper part of the ramp area.
This is a map famous for the very prominent availability of the so-called "Noob Combo". A good trick is to collect as many plasma grenades as you can, then hang in one of the two huge windows near the bottomless pits (where the gas canisters drop into) and blast everyone who shows up. The gas canisters provide a high advantage, if you see a filled one and somebody standing near it, simply by firing at the canister can cause it to explode and kill anyone near it. That means that if someone is shooting at it, you run away.
Colossus is an oversize map but there isn't much breathing room. Snipers will most likely camp on the opposite side of the map and pick you off, either with a distanced weapon, Battle Rifle, Covenant Carbine or long-ranged Sniper Rifles. Don't let this happen. Know the course, and know the paths that will block their fire, or at least let you get your shields up before dying.
The walkway near the Gravity lift is a popular battle spot. Check around corners and prepare for the worst anytime you're near there, and make sure to watch your back/each other's backs.
The mid-sized windows near each ramp to the aforementioned walkway can be jumped through, for surprise or quick maneuvering.
On the previous note, expect grenades to be thrown through them, and do so yourself. If there's a blip on your radar, or if there's someone that just went up a ramp, don't hesitate to toss a grenade or two up through them - it could mean the difference between full shield and no shield.
Sniper rifles are commonly used on this map. Don't hesitate to grab them, as they're infinitely useful.
Use the explosive containers to your advantage; shoot them when enemies are close to them.