Harder Kill Achievements
Confederacy of Crunches
This one is fun but hard, as you have to go through a whole level (any level) with only melee weapons - so I would NOT recommend doing this one in Realism mode at any time. The easiest way to complete this one is to not pick up any guns at all, and don't use your pistol even if you get into real trouble. Otherwise you have to start all over, but only you have to use the melee weapons - others in your group can use guns. You can throw things when you do this Xbox 360 achievement, just nothing gun-wise. The easiest level to use is Dead Center.
You should be able to start on this one while you're doing Confederacy of Crunches and throughout the game, as it means that you have to decapitate 200 infected with only a melee weapon - no guns. The best melee weapon to do this one with is the baseball bat, even though it's not any type of knife.
During Dark Carnival, you need to find a place where a clown is leading 10 or more normal mobs around and burn them with a molotov. So, when you're on this level, just lob your molotovs at clowns when there are lots of mobs around them.
Level a Charge
This one is a bit hard to do solo, so if you can get a friend to play with you do so. You have to take out a charger with only a melee weapon while they're charging. So, the best way to do it is to time it right so that you sidestep a charger while they are coming at you and then whack them as they go past. The best thing to do is to have someone shooting at them while you're doing this so you can get the two major hits it takes to kill one.
This one is gross but not too hard if you're a good shot as all you have to do it take out a Spitter before she can spit on you. So, just shoot every one of them until you get it - which should be right away.
A Ride Denied
This is a funny one, as you have to deal with the Jockey special infected. Simply kill one in the first 2 seconds of one of these guys jumping on one of the survivors. This is another one of the Xbox 360 achievements that you should easily get.
Violence in Silence
To do this one, you have to go through the impound lot and then get to the safe room in the cemetary without setting off any of the alarms along the way. So, this is where you'll want the computer players with you as they all have perfect aim. Go through here, using melee weapons, in Parish level, without jumpin on any of the orange colored or smoked into any of the cars. Get to the safe room and you'll get the achievement.
Bridge Over Trebled Slaughter
This is done in the finale of Parish - the final level. To do this one, you have to get across the bridge in less than three minutes, which is pretty darn hard. So, you need to have all human players here - no computer controlled guys (or gals). Then, when the bridge drops, run your butts off for the other end. You will need tons of adrenaline here, but don't use it yet - you want to use it toward the end of the bridge when the mobs will try to swarm you and you don't want to use the immunity that you get to the infected's slow down just yet. Now, there is a tank halfway across, which is the hardest part and if you can't avoid him, it'll cost you the Xbox 360 achievement here. Get around him somehow - don't fight him or get hit. If you find that you're running out of adrenaline, there are several on the trucks as you go, so you can use the few seconds it takes to get them if you need them.