First, I would like to point out that ThatAverageGatsby has a
fantastic video showing how to complete the Collector Ship platforms with an Engineer using two different methods. If you are having trouble, the first method he uses by utilizing the Avalanche is great and I cannot endorse it enough.
And I cannot stress it enough: if you are having problems,
use the Avalanche.
Use the Avalanche!
Another thing to note, is how the Collector platforms spawn:
1) When you start, you will two platforms, one will have Collectors and will land in front of you. The other one will have a Scion and land behind the Collector platform and will move left and right.
2) When one platform s cleared of enemies, another platform will land to the left. Another Scion will land in behind it.
3) When you kill a Scion, another platform will land on the left again. This will be the last platform.
What I usually do:
1) Stick a Drone one Scion so he does not take shots at you.
2) Kill the Collectors. If I have the Avalanche, I freeze everybody then run in and take them out.
3) When the next Collector platform comes in, I freeze everybody again, then kill them all. If Harbinger assumes control before I can freeze everyone, I try to kill the other Collectors first while dodging his attacks. While Harbinger can kill you, his attacks are most dangerous when other enemies are around since his attacks can stun you or knock you out of cover, leaving you exposed to other enemies who can kill you rather quickly.
4) When both Scions are left, I whittle down their armor then try to kill them both within a few seconds of each other. Removing both Scions before the other platform arrives makes it easier to deal with the Collectors.
5) Same as before, use the Avalanche to freeze, the kill. If Harbinger spawns, cast your Drone on him.
If you do not use the Avalanche, you the Arc Projector to strip barriers and then useyour own/Mordin's Cryo Blast and/or a squadmate's Pull or other disabling power to disable the Collectores then shoot them dead. Frozen, Pulled, and Ragdolled enemies take more weapon damage.
If you do want to use any heavy weapons (you are saving the Cain for the Praetorian for example), then it becomes a little tricker. I would still cast your Drone on the first Scion. Having Miranda and Thane for their Warp helps strip Barriers quickly so you can disable them with Cryo Blast or Neural Shockwave; use your SMG to shoot their barriers off.
Good squadmates:
Miranda: Warp
Thane: Warp, Throw
Jack: Pull, Shockwave
Jacob: Incendiary Ammo, Pull
Kasumi: Flashbang Grenade (prevents Harbinger from casting his powers for about 20-30 seconds; Flashbang requires loyalty)
Samara: Pull, Throw, Reave (Reave requires loyalty)
Mordin: Cryo Blast, Incinerate
Bad squadmates:
Grunt
Tali
Zaeed *
Garrus * (sorry capn233)
Garrus and Zaeed have Concussive Shot which has a multiplier against barriers and can knock down unprotected enemies, but there are better squadmates.
You can beat this section with any combination of squadmates as long as you use good tactics.