What i did is use the pistol, then choose chapter 4 - the dig on very eay, when you get to the grenade bit just as it tells you, after that just keep running and shooting the grey people once then hit square try to avoid the black ones as they tend to take 2-3 hits with pistol.
(if you think you got ten and the trophy didnt pop up restart the chapter and start again............ i thought i must have had about 20 times but i just kept on repeating it and eventually it popped up)
24 hours on halo pc, and about 30 hours on avp pc, back in the day. These days i can barely manage 5 hours on MW2 before me back starts to aches and i just get bored!
The most Ive done was, about a day an a half. BUT!! I woke up at 11am, went to the beach until 4pm, got home around 630pm, played beer pong until about 4am, AND stayed up the rest of the day.