I unfortunately don't just have one really troublesome Platinum Trophy. I'll list them off and the reasons why they were so tough.
Killzone 2: I didn't struggle as badly as I would have, if I was working on the miltiplayer solo. Thankfully I was in a boosting group that made sure we all got that coveted Platinum Trophy. What made this so troublesome for me, was the main campaign on Elite Difficulty. Your character might as well be wearing paper armour for all the good it does on this difficulty. I bit the bullet more times than I care to remember on this brutal difficulty.
Warhammer 40,000: Space Marine: I found boosting partners who also had bots or additional consoles/accounts. Together we grinded out all the Online trophies, excluding the DLC. Though I did finish a few of those trophies too. The troublesome part was the grind, the monotonous endless grind for kills, to achieve 40,000 kills across all game modes.Chaos Invasion Arenas did help with the grind, but it was a soul-draining experience.
Resistance 2: The campaign was a cakewalk, it was the online grind that once again, made this trophy so troublesome. Unlike Space Marine, it was only online kills and I am not a great player when it comes to online FPS games. Or at least Resistance 2's. What was worse, is that I got messages from what I can only assume were 12 year olds or people with nothing better to do, criticising my skills. In truth I was just doing the bare basics and getting a 100 kills a day. Grinding out until nearly the last hour, when the servers closed.
Surgeon Simulator: This was the most troublesome game of them all. This game made my hands hurt and strained my eyes from the amount of focus and precision that was needed in this game. Bearing in mind this is a goofy physics simulator-type game where you perform delicate invasive surgery. I was stuck on one level for months, getting the highest rank on every surgery was brutal. You have to perform surgery in a moving ambulance and a moving gurney multiple times for different surgeries. It is however one of my proudest platinum trophies, but it was ball-twistingly frustrating to complete this game.