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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.
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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.
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Fight to the Tower
Move out and take cover behind a wall as soon as possible. There are a ton of collectors here and you should spot the harbinger at least once. There should also be a few husks just to make things annoying. The biggest problem is that there are also two scions. These mechs can do the shockwave attack that Jack does. If you are not crouched behind cover when it hits, it will kill your shield and knock you out of cover. It's really dangerous.
I suggest you just ignore them at first. They shouldn't be in a position to do many attacks. Take the break at the beginning to kill the collectors. By themselves the scions aren't too tough. Just use a heavy pistol to tear them down along with incinerate.
Search the area. There should be one more dead collector you can search for some biotic research. There are a few safes to check too. You need to bypass the door in back to get to the next area. The relay tower is right in the center. There should be a scion here along with some husks. Just melee them if they get close. Take cover behind the boxes and take the scion down. Gather up some ammo. There are more power cells over in a box on the side building.
I suggest that you save now. The next one can go really wrong really fast.
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Holding the Tower
We need to hold our ground while EDI hacks the relay tower and gets the cannons working again. The collectors don't really like this, so we've got to hold them back. The first wave comes from the back wall farthest from the entrance. Take cover behind the the crates and get a good line of sight on the back right corner. Make sure that you're not farther up than the relay tower. Some will come over the wall and take up positions in the alcove in the middle of the right wall, so you don't want to get ahead of that.
Just hunker down and take them out. You shouldn't really have any trouble. They're just average collectors with the occasional Harbinger, but you should have plenty of distance and cover. After the last one dies, someone in your squad should warn that more are coming. You've got about fifteen seconds to gather up more ammo before they hit again. They'll come in from the entrance this time though.
Just hunker down and hold your ground. There's no trick to this other than picking them off one at a time and using unity to bring back any of your fallen squad. The third wave is the fun one though. They've sent in a praetorian to take us down.
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This massive fighter is really tough. It can use a beam attack that will just shred any shield. Plus, once you take down its barrier you only have a limited amount of time to knock down its armor. After a certain point it will stop, jump into the air, and crash down. It's invulnerable during this time so hold your fire. It will then restore its barrier and start the process all over again.
Thankfully you're listening to me, so you should have a full particle beam weapon on hand. We're just going to use this for a quick takedown. Get behind some cover and just play chicken with it. Your beam weapon should do more damage than it's attack, so you should be able to just take the hit for a few seconds. Watch your shields and health though and duck back down if it really starts to hurt you. We're just going to rip through its shield with the particle beam and then focus right on the top protrusion. This seems to cause extra damage. If you're lucky, you'll get about half of its health the first time. Hold your fire and wait for it to recover and then do it again. I got it down in three bursts with this method and barely took any damage after the first standoff.
Once the praetorian dies, the collector ship will bug out. You will get a chance to apologize to or berate the mechanic. We'll also meet an old friend. Either Kaiden or Ashley will be here depending on your choices in Mass Effect 1. If this is a new game, then it will just be the opposite of your gender. Go ahead and talk to them. Note that the outcome is always the same. They will refuse to work with you or Cerebus and storm off. If they were an old romantic interest, then they'll send an apology email later. There is no way to recruit Ashley or Kaiden.
Give your report to the Illusive Man and accept three new dossiers. We've got more recruiting to do and several loyalty missions to start.