heady plover
I already did this platinum on the PS3 with 2 friends. There's a way to do it solo with 3 consoles but it's not that easy, anyway, I have 2 Xbox 360 ready for some hours to boost and I can do many things solo or help you with it, but there are two achievements that are for 3 people.
You can add me and text me over there, my gamertag is ChrisHenares and I'm ready to roll every afternoon and night this week. Having 2 jobs is hard to get these online achievements but I got a week off and the time is now.
On that mission where you chase some missile, you get killed and restart the checkpoint. 2 consoles + 1 more guy aiming the missile with DRM and finally destroying it will do the job for that last achievement.
Then for the 50 hours of multiplayer gameplay it's quite easy and conformtable because you only have to go to Mission 2, right at the start do nothing, there'll be a countdown to 59:59min (1 hour) and after that you get a GAME OVER screen. one of the two players must hit the A button to restart, there's no need for both players to do it, just have a good network.
There are not so many games that "helps" the players for multiplayer achievements/trophies like this, so it's good to try it.