Welcome to Zombieville!
Difficulty: (0/10)
Just die. You'll get it without trying.

A Fistful of Zombies
Difficulty: (0/10)
Grind 1000 kills. You'll get it after your first few games.

A Few Zombies More
Difficulty: (0/10)
Grind 10000 kills. You'll get it eventually.

The Good, The Bad, and The Undead
Difficulty: (1/10)
Grind 30000 kills. This one takes a minute, but hey, every second you play, you're working toward it.

Lifting the Curse
Difficulty: (1/10)
Defeat 50 cursed zombies. The tanks can be tricky and burn through your ammo. The best way to tackle this is with melee or explosives. Anything else is going to drain your ammo fast.

I Won't Miss, Ultimate Bada**, Mad Dog, Weird Science, 4th of July, Pistols at Dusk, Buckshot Bourgeoisie
Difficulty: (3/10)
Unlock the special guns. Grind kills with each weapon type. It'll take you a dozen hours or so. The achievements won't unlock until you purchase the weapon(s) in the shop.

Easy Money, Mad Stacks, Wolf of Z Street
Difficulty: (3/10)
Collect $5000, $25,000, and $100,000 respectively in a single run. Use melee and activate the money-making perks. The last one can be hard if you aren't leveled up.

Survivor
Difficulty: (1/10)
Reach infection level/wave 10. Get a few upgrades first and it's easy.

Immortal
Difficulty: (3/10)
Reach level/wave 20. You're going to need better weapons and some perks before you can reach this level, but it's easy enough once you do.

Master of Unboxing
Difficulty: (0/10)
Open 250 weapon cases. Just grind it.

I Got 5 On It
Difficulty: (0/10)
Upgrade 5 different weapons to +5 (yellow). A matter of time.

Gigabrain
Difficulty: (0/10)
Upgrade perks 15 times. If you're just achievement grinding, then upgrade each perk once, as it's cheaper that way. Otherwise, get it naturally over time by upgrading the perks you want (such as the money makers).

Lord of War
Difficulty: (1/10)
Sell 200 weapons. It's easier to sell the low-ranking weapons that spawn in the first few waves. The higher waves cause weapons up to +10 to spawn and you might want to keep those.

Confident, or Careless?
Difficulty: (1/10)
Sell a +6 weapon or higher. Reach wave 5 and one will eventually spawn. Cursed zombies have a good chance of dropping one.

The Chosen One
Difficulty: (2/10)
Level a weapon to +10. I highly suggest focusing on the Flame Edge or Megablade for their money-making abilities, and the fact that they never run out of ammo, or go with your favorite. It'll take a good bit of time to earn that much money, though.

Go, Team!
Difficulty: (1/10)
Unlock ten characters. Find ten tokens and spend them. Ten is no problem... you'll find them naturally.

More Problems
Difficulty: (2/10)
Earn $1,000,000. It just takes time. Again, the money-making perks and using melee help a lot.

I'll Save You!
Difficulty: (1/10)
Freeze a burning enemy. Flamethrower/Molotov/Flame Edge + Cryogun/Megablade. Wait until later waves when you can't one-hit enemies.

Juggler
Difficulty: (3/10)
25 kills with 10 different weapons. Remember the four you take into battle with you, then find six different ones. Best done at the beginning of the game, when zombies are weaker and you have enough ammo to take down 25. Keep a count in your head and count to 30 just to be safe, as you get no feedback until the achievement pops.

Treasure Hunter
Difficulty: (4/10)
Find all 40 tokens. This also unlocks the Megablade. The map isn't that large. Hug the outer walls of the map at first. Pick a direction and run that way for the entire game. Check backyards and enclaves.

Ultimate Arsenal
Difficulty: (5/10)
Unlock all weapons. This includes the Megablade, so you need to find the tokens and grind all the individual weapons first.

Smooth Operator
Difficulty: (5/10)
Survive until wave 11 without getting hit, while killing at least 250 zombies. Use a long-range melee weapon and explosive projectiles when things get crowded. Be aware of zombies popping up beside you and stray acid balls. You don't want to depend on guns with ammo as you might run out at the worst possible moment. Find an "arena" (a large open space) and focus on the most dangerous zombies first. Make sure not to get hit by exploding zombies. Leave the normal zombies alone and let them come to you while you deal with runners and spitters.

Overall Difficulty: 3/10
Lots of time-consuming achievements, and Smooth Operator can be a headache.