Red Faction: Guerilla
Let me notify you that NO ONE plays this game online, and if it's one of those magical days where people are actually playing, they are pro and in a party, making them unbeatable. The trophies require: 250 matchmaking games played (which is kinda hard, since the match making is dreadfull, after a set amount of time it searches for a new game). After that you actually need 250 wins... The other trophies require 5000 kills and alot of XP, those are boostable. The plays and wins require matchmaking. Right now I'm at around 60 something wins and 110 games played or something. Also the collectibles in this game are overkill, 300 ores? Lord have mercy
GTA 4
I'm boosting the Auf Wiedersehen Petrovich with my cousin, but even then it's taking a lot of time. Almost nobody is racing legit in GTA, and since there are like 30 races (not to mention the GTA race mode, which makes 60) it's kinda a pain. Even finding some team games is hard, I still need my cops 'n crooks wins for instance. And like RF:G the 200 pidgeons are insane, still haven't even botherd. Just by playing the game normally I found 3.. 3 out of 200. These bastards must be insanely hidden!
Modnation Racers
The amount of creation XP, wins and stuff is insane. If you can't make yourself a good name soon, you won't get much experience. 100 wins is kinda hard aswell, also top 10% of the hot lap at a day.
Killzone 2
This platinum trophy requires a lot of dedication. I managed to do this, because I "was" (tried it a few weeks ago and now I suck again

) terrible good at it. This game requires you to be in the leaderbords at the top 1% of that week. That's crazy! Imagine there are 50,000 people playing, you would have to be in the top 500.
I can't think of anything more atm, but my point is some trophies are really getting on my nerves