Ah, I've been playing Resident Evil 5 all weekend. It was so much fun. 
I beat the game once on co-op on normal in 10 hours and beat all the mercenary stages once on co-op. Next I will do Veteran and then Professional difficulty. I would have played Veteran first for the increased challenge, but I want to get the most plays out of this game b/c I enjoy it so much. RE4 is one of my favorite games of all time.
@mattbfg
You can get all the trophies by playing co-op except the very last one I believe. The one where you have to get Sheva to trust you.
@Jboy0990
To save the special someone you have to "work together". (SPOILERS) One person has to "hold" that special someone while the other has to "pull". You have to be standing. The trophy will not unlock if one person pulls from the ground. I didn't get it my first time either for this reason. Just keep pulling from the standing hold and you'll get it.
@KylieDog
You can get the stab the heart trophy with Chris. He pulls out his knife for a ground stab when hovering over that enemy type while its on its back. Just keep replaying sections with those creatures to get it.
Essay Pro