Geth Colossus

Mass Effect
The Geth Colossus is similar to the Geth Armature, but much larger with more health, stronger armor and better weaponry. They are distinctive from Armatures because their armor is brighter, somewhat platinum-silver in appearance, with ridges on the back. They are equipped with a machine-gun style weapon and a powerful Siege Pulse. They are the largest of the geth ground forces.

Tactics
The Geth Colossus is a monumentally dangerous foe, but fortunately they are almost always encountered whilst in the Mako. So long as you stay at long range or circle strafe using the main cannon, four or five shots will bring it down once its shields are gone. Like Geth Armatures, before firing its Siege Pulse, the Colossus will bring its legs together, making it easy to avoid if you use the Mako's propulsion jets. If it becomes necessary to engage one at close range, repeatedly ramming it with the Mako to keep it from getting into its firing position can be an effective strategy. However, this leaves little or no time to engage other enemies and is therefore extremely dangerous unless the Colossus is alone.

If you want a huge XP award, it is possible to engage a Colossus on foot with a well-equipped and balanced team, but your talents and weapons must be high level to pull it off. First of all, you must keep your shields charged with Shield Boost or Barrier, otherwise the Colossus' secondary weapon will shred them, then kill you. Switch to Tungsten Ammunition and use either an assault rifle or shotgun to attack it. Don't try AI Hacking because the Colossus is just too powerful - save that for any nearby lesser geth to erode the Colossus' shields instead, and use Warp to soften it up. Also, bringing it down to near death with the Mako and then hopping out to finish is a favorable tactic.

Hit the Colossus with Lift or Singularity if you can, bearing in mind that the Colossus is so heavy that only a maxed-out Lift/Singularity talent or a combined Lift will get it off the ground. Without Lift, tech abilities like Sabotage and Damping are life savers to stop it from firing or using its Siege Pulse against you. If you're out in the open, keep moving and spread your team out if you can. Cover is a bonus, but while Geth Armatures will usually just stand there and fire in your direction if you leave their line of sight, a Colossus will actually give chase once you begin your attack, so bear that in mind if you have to fall back.

Another option for defeating a Colossus and gain a vast amount of XP is as follows, shoot the Colossus with the Mako until it's down to about 1/6 hit points and no shields. At this point ram the Colossus and park the Mako on top of it. If you executed this correctly the Colossus will be unable to stand up for quite a while, giving you ample time to finish it off. Another tactic is to attack the Colossus from long-range with the Mako, while inside an area with decent cover where you can hide the Mako from the Colossus' attacks. Once you have gotten the Colossus down to about 1/6 health or so, put the Mako in a safe place (behind a wall or on the other side of a hill from the Colossus) and send your squad members to the same hidden/protected area by using the squad commands. Equip Shepard's sniper rifle, and fire away. If you have done this correctly, and your sniper rifle is reasonably strong, it should only take a few hits to kill the Colossus.

Mass Effect 2
There is one Geth Colossus present on Haestrom, which acts to serve as a level boss at the end of the mission to recruit Tali'Zorah vas Neema. A notable trait with this particular Colossus is that it is the Armature steel gray color, as opposed to the typical Colossus white. Interestingly, the quarian marine Kal'Reegar calls the Colossus an 'Armature class' unit, indicating that at least the quarians see it as a variation on the standard Geth Armature.

There are also multiple Colossi on the Firewalker Pack, but they are fought with the Hammerhead and lack the self-repair mechanism, making them much less challenging.

Capabilities
Offensive

The Colossus possesses a long-range siege pulse weapon and short-range mass accelerator machine guns. It should also be noted that the shots from the Colossus's main gun home in on whatever it's aiming at. This can make the fight very difficult. However, unlike in the original Mass Effect, this siege pulse will not kill in one hit, even on Insanity.

Defensive

The Colossus on Haestrom has very strong shielding and armour, and it can self-repair, recharging its shields.

Tactics

 * The Geth Colossus is protected with heavy shielding and armor. After destroying its shields, unload with heavy weapons and armor-damaging powers such as Incinerate or Warp. Once enough damage is done to its armor, it will collapse into a defensive position, enter into a self-repair mode and rapidly regenerate its shields. Fortunately, damage done to its armor is not repaired. When it enters its repair mode, wait until it stands back up before opening fire again.
 * A good way to defeat the Colossus is to bring its shield down to half, then use M-920 Cain to destroy it from across the field. On lower difficulties, it is not even necessary to bring down its shield to destroy it with the Cain.
 * If you don't have the M-920 Cain, just make your way up the right side all the way to the area right on the side of the Colossus. Then just chip away at the shields with Overload. Don't worry, once you're at this spot the Colossus won't repair itself. Once you make it down to the armor just use whatever heavy weapon you have to take it down. The M-98 Widow and M-96 Mattock work fine as well, more so on lower difficulties. Or just keep chipping away at it with Incinerate or Warp.
 * The Colossus is unaffected by Stasis.
 * The Colossus can see through Tactical Cloak.