First time was way back just when Platinum was added. I searched for a Silver Match and for some reason ended up in the mid of Plat Wave 1; playing my Krentinel (Well, any krogan really) for the second time and my best weapon back then was a Phaeston 3.
It was White/Cerberus, and everybody stood behind the counters in the lower room, grabbing evrything and shooting the Banshees and Atlases from afar. I could barely understand what was going on, which was aggravated by the other 3 players talking a lot, and really fast in French (the only french words I know are "croissant", "abajour" and "Sophie Marceau").
Yet, we managed to last to wave 6 post objective; back then it made me happy for getting 60K so fast (I was breaking to silver, then).
Muuuuuch later, about november or december last year, when I was managing to extract in gold somewhat successfully, I heard about the box of shame and decided to try it and see if I could handle it. Ended up just going into the box twice in the whole match, although the whole team was constantly around it. It was a good team, and it helped build confidence, so from them to know I have been playing the occasional plat, though there are few maps where I manage to really contribute (Rio, obviously, Giant, which is the easiest map for me in any difficult, Glacier and White).
Another thing that helped me was the Lodestar chalenge; I managed reapers, cerberus and collectors in teh first; but it took me about ten games to successfully extract against the Geth (and I only did so due to lag, because I was down!). Platinum became somewhat easier after that, mainly because I learned the number one rule of staying calm.