The challenge of this level is dividing your attention between the Reaper and the Collectors. It's important to scrap each Collector wave quickly, so you can focus fire on the Reaper.
Dealing with the Collectors:
You'll want to bring squadmates with armor an barrier stripping, and that give you good AOE options. I'm going to assume you don't have Slam for priming Warp bombs. In this case Samara + Thane is a pretty good combo, since on Hardcore Samara can strip barriers off Collectors (Area Reave), and Shep can freeze them with Full Cryo. You can also weapon-strip a Collector, prime with Samara's Pull, and detonate with Thane's Unstable Warp. Give Samara a Vindicator and Thane an Incisor, and they can actually do a fair amount of weapon damage as well. This combo also allows you to focus fire on Harbi when he shows up with triple combination Incinerate (Shep), Warp (Thane) and Reave (Samara).
By constantly distracting Harbi with a Drone you should be able to prevent him from staggering you, and keep fire off you and your squad. I've also seen players use Grunt + medigel as another quasi-drone (with the added benefit of Squad Incendiary Ammo).
Before the Reaper fires it's red energy weapon, you'll see the energy building up in the area around it's mouth. Keep an eye on this, and get into cover before it launches. Once it blows it's wad, you can maneuver again and re-engage the Collectors. Try to keep your squad behind you to avoid delays in power delivery, and keep the Collectors focused on you and/or the Drone. If your squad mates do go down, take advantage of the cool down reset when you heal them with medigel.
Dealing with the Reaper:
As I recall the Reaper is invulnerable to weapon fire except in the orange areas around its eyes, which are classified as armor for purposes of damage calculation. Between waves of Collectors (and the Reaper energy weapon), just keep hammering on these areas with armor-damaging weapons and auto-aim powers (Warp, Reave, Incinerate). At certain points in the animation cycle the Reaper moves around randomly making the eyes very difficult to hit. Since my aim sucks, I actually just wait out this movement so as to avoid wasting ammo, and then re-engage when it is holding fairly still. Keep an eye on ammo count, and remember to collect the ammo off dead Collectors after each wave.
The Cain is also pretty good against the Reaper, but be sure to plan and and time your shot carefully. The Cain has a very long build up period before it fires, and during this time the Reaper can easily move away, and you are vulnerable to the Reaper and Collectors while you are out of cover taking the shot. I personally prefer taking it down the old-fashioned way.
Good luck!