It's the Collector! - 20G
Collect all the projectiles.
Here is a list of the levels and what weapons you can find on them. In addition to those weapons, there are four that you receive for picking up the final four hidden case files. There are hidden case files stashed in environmental destructible objects on all levels(garbage cans, hotdog carts, potted trees, mailboxes, etc) with the exception of the boss fights in cases 4,8 and 11. They spawn randomly so you won't find a hidden case file every time you complete a level, don't get discouraged if you miss them on your first couple times through. You can tell which levels you have hidden case files collected for by highlighting the case on the case select screen and if there is a file folder icon in the bottom left corner of the case file then you have that level's hidden file.
You can also have another player drop the weapons they are using if you need it, you don't need to complete the level to save either, just picking up the weapon gives you credit for having it.
Liquor Bottle (5th hidden case file)
Zombie Arms (6th hidden case file)
Demon Head (7th hidden case file)
Wedding Cake (8th hidden case file)
Hammer (only ever found 1 hammer in the level)
My New Favorite Number - 15G
Defeat 50 enemies in a row without getting hit.
I thought for sure that this achievement would be the hardest in the game but it really isn't. If you follow my general advice listed under See You In Hell! you should have it on the first or second level. If you are having difficulties try running through the second level as the zombies there are pretty slow and you shouldn't encounter anyone shooting projectiles at you.
Repeat, Repeat, Repeat - 10G
Use any special attack 100 times.
Simply use any special attack 100 times just load level 1 or 2, sit on the first screen where there aren't any enemies and keep casting special attacks with the
until the achievement pops. Leonard paired with Demon Baby Gustav seems to be the best choice for this as he has the highest innate special attack recharge and Gustav just makes it better.
Wakey, Wakey, Eggs and Bakey! - 10G
Resuscitate 50 allies.
This achievement can be done either online in co-op or locally with a second controller. Best bet is to load up a guest account and either run them continuously into the lava fountains in the later levels or load up a boss fight and let the boss kill your spare character and just keep reviving them with the
Keep These Bad Boys Comin' - 15G
Rack up a 20 score multiplier.
You just need to get up to a 20x score multiplier by defeating enemies while minimizing the amount of times you are hit. Each time you are hit the meter decreases a bit. Almost any level should net you this achievement if you are good at avoiding hits, the best is probably level 2 for the decent amount of enemies and ease of avoiding them. If level 2 is no good for you try level 3, there are more enemies but it's harder to avoid being hit by all the projectiles.
Put Your Cheeks Into It! - 10G
Kill 1000 Manbirds.
Simply kill 1000 Manbirds, these are Humanoid enemies with bird heads and wings. You should have enough kills by the time you finish the story but if you don't, the best places to grind Manbird kills are levels 1 and 6.
Dirty Leg Draggers... - 10G
Kill 300 Zombies.
Just kill 300 zombies, one of the few places you will find them is Case 2. It shouldn't take more than two runs through this case to get enough kills, so don't worry about it as you will likely get it while trying to get the hidden case file here.
No Loitering! - 10G
Kill 5000 Demons.
Killing 5000 demons can take a while, luckily they are found in almost every level and in great quantities. Anything other than a manbird or zombie is classified as a demon which is more than 75% of the enemies in the game. If you don't have this achievement by the time you are done with every other achievement I find the best places to grind are Cases 3 or 5 as the enemies there have a lot less health than their counterparts in the later cases.