My first Platinum was for Lodestar.
I posted in a thread that someone had started to help non-platinum players get groups to do the challenge. Except no one was actually helping us. I ended up grouping with three other people who had never touched Platinum before. We decided to go for Reapers first, and picked Giant, hoping to camp the control room. Amazingly, it worked! I was playing a Vorcha Soldier, there was a Justicar, I think a HEngie, and some kind of infiltrator. We spent the whole match running circles around those two ramps leading from the control room into the room behind it with the ammo box. I don't remember what the objectives were, but I do remember that we were inches from wiping on all three waves.
I was easily the worst player out of the four of us (I'd almost never even played gold yet at that point), but I do remember being left to clutch the wave 10 cleanup by myself. 4 panicked missiles, 3 ops packs, and 4 gels later, the enemies were dead, and we eventually managed full extraction.
Then we did the exact same thing for the other factions, though Cerberus and Geth didn't go nearly as well, and took multiple attempts each. Looking back, trying to hold the line with that many Hunters and Pyros, or Phantoms, all in the room with us was probably a terrible idea!
I've tried the occasional Platinum pug since then, but I've almost never reached extraction without another BSNer in the match. Specifically, FatherOfPearl. I landed in his lobby 4 days in a row. Plus, the Banshee/Praetorian/Phantom spam tends to bore me really fast since I mostly enjoy power-spamming classes.