2-6: Highway - Army of Two: The 40th Day
You exit out onto a section of elevated highway - and there are enemy here to be killed. As you move along the highway here you will notice that the enemy here seem to cluster together a lot. This is a good opportunity for you to pick up the Pumpkin Patch Achievement / Trophy for shooting two enemy in the head with the same bullet. Basically you get several chances to do this if you flank the enemy positions behind the Jersey barriers.
Collect the fourth cat
You will reach an ambulance that is blocking your way here - and you co-op push it down the ramp and engage the enemy below. This is a good time to pull out your sniper rifle and use it to good effect. As you reach the road below you will notice a green highway sign with a cat face on it - immediately to the left of the cat face is another one of the Cats (Neko Cat 4 of 12) that we need to shoot! Now clear out the rest of the mercenaries here, and proceed down the road.
Taking down a Chaingun Boss
You will now get a brief CS that shows you a boss mob - a heavily armored mercenary wielding a Vulcan Chaingun. Nasty! Concentrate your fire on the tank on its back until it explodes, killing him, then run up and grab his gun for a little chaingun action of your own! Granted you only get to use it on a pair of regular mercenaries but still, sweet!
Ahead and to the right is the door that marks the exit for this chapter - head on over there and hit your B-key to exit and get your stats. You should have saved 3 out of 3 civilians, and shot the 2 cats for this chapter as well as the two radios.