Army of Two 40th Day - Hostages & Ammo Crates
When saving hostages you must kill all the enemies before they assassinate the hostage, not always easy however. You can go in all guns blazing & hope you kill all the enemies before they kill the hostages, this may work but usually the hostage ends up dying either by being assassinated or caught in the crossfire. The best tactic is to use mock surrender which buys you a few more seconds for your co-op partner to get a good spot to start picking a few off & then finish off the left overs.
Similar scenario with ammo crates you have to kill all the enemies before they can get to the crate to lock it shut. Once it's shut you cannot open it.
Luckily today Abbas & I managed to complete Chapter 3 saving all the hostages & collecting all the loot from the ammo crates (Yes we did have to try a few of them again & again & again .....& again)
I had fun trying to get 100 sniper kills (cue quote from Abbas "Wow !! you are really good at headshots dad") to unlock that achievement, whilst Abbas went for the 400 kills with Assault Rifles.