Mass Effect 2 Walkthrough - Main Quest: Horizon

You should get the call to go to Horizon after you recruit the first batch of people. After that happens, Kelly will tell you to go talk to The Illusive Man in the briefing room. He'll explain that situation on Horizon and you'll automatically go there. 

This will be our first run in with the collectors. They use a lot of barriers and they have a few tough mechs with them. I suggest that you bring at least one person with incinerate to deal with the mechs and the armor. Mordin is a good choice for now. Jack is a good choice for tearing through the collectors once you take out their barriers. Grunt should be a fine all around soldier and he is great for dealing with the few groups of husks. So pick the two that complement your character best and then head out.

Go forward and around to the first area. A group of collectors will fly into the clearing and drop in to start the attack. Take cover behind the wall and just wear them down. Don't worry. If you could handle the Blue Suns, you should be fine with them. Use something that fires fast to wipe out their barriers and then take them down with the heavy stuff.

Go ahead and search the area. There should be a PDA to hack and you can check the housing unit nearby for more credits. Nothing else is too interesting though. Go up to the next area to trigger another attack. They have a few husks with them, so be careful. If Grunt is with you, then he'll probably handle them just fine. Search the area and look for a dead collector. He should be right along the path. Examining him should give you the heavy skin weave technology.

If you're lost, you need to go all the way to the far wall and look behind the housing units. There should be a diagonal path to the next section of the colony.

This is where things get a little interesting though. The cutscene now introduces our biggest enemy. Harbinger can take over a collector's body and turn them into supersoldier. The takeovers seem to be somewhat random in who they take, although he does favor claiming collectors who are about to die. He will only do takeovers a set number of times in a battle apparently. 

Harbinger is easy to spot since he glows. He should just start walking toward you and fire off a biotic attack as he moves. You need to just treat him like a usual enemy. His barriers go down quickly to submachine gun fire and a heavy pistol will just tear right through his armor. He doesn't have any actual health, so once the armor is gone he disintegrates.

There is plenty of cover though, so you can just take cover and wear them down like usual. This is more of a training fight to introduce the harbinger anyway. I do suggest that you pay attention though. Collector assassins with particle beams should start showing up now and they can really pack a punch.

The good news is that we can get one of those guns right now. Go through the area and approach the stairs. You should see a person in stasis. They are your marker. Note that you can't help them directly (although we are saving them by driving the collectors back). Look around and you should see a particle beam gun lying against a tree. Pick it up. This is an awesome heavy weapon and it's great for tearing right through barriers and armor. If you have enough ammo, it will also make the final fight incredibly easy. So don't just waste it on Harbinger.

You can search the housing units for some credits. When you're done bypass the door at the end to gain access to the garage. There's a survivor inside who can tell you what happened. He'll open the doors to the inner relay tower and our final few fights. Search the safe for some more credits before you go. There should also be a medical pack and some minerals.