There really aren't many "difficult" trophies, the four most difficult trophies by far for KZ2 are the Valor medals, which require you to place in the top 10%, 6%, 3% and 1% of the multiplayer rankings for that week. There are also trophies which require you to get intel, but a few Youtube videos should help with that. All the other trophies are the average " Kill [x] amount of Helghast" or "Complete [campaign level] on [difficulty]".
The entire list is right here.