The achievements listed below are those that can be done from the start of a new game, continue an old savegame or inputting a password. If your playing in one player mode/single player, you can use your AI special ability and it will count for the specific ability you have used. For two players, I have not tested it but I believe it should work.


Huntsman
Kill 500 enemies in a single play through
-Kill any enemies will do for 500 times

Super Frog Killer
Kill 500 Frogs
-You can only find frogs in World 1, level 2 and 3. Kill frogs 500 times.

Specialist
Use 50 Specials from your inventory
-Simply use any special ability 50 times.

Bonfire
Use the Molotov Special from your inventory 50 times
-Thug start with Molotov, Brigand require a butt load of money to upgrade his stats and purchase it. Use 50 times Molotov.

Be Gone!
Use the Shotburst Special from your inventory 50 times
-Brigand start with Shotburst, Navvie need upgrades and purchase Shortburst. Use Shortburst 50 times.

Death from below
Use the Ground Mine Special from your inventory 50 times
-Only Mercenary can purchase Ground Mine. Use Ground Mine 50 times.

BOOM!
Use the Bomb Special from your inventory 50 times
-Mercenary start with Bomb. Use Bomb 50 times.

BLAST!
Use the Dynamite Special from your inventory 50 times
-Navvie start with Dynamite. Use Dynamite 50 times.

Back to the ice age
Use the Freeze Monster Special from your inventory 50 times
-Only Preacher/Scientist can purchase Freeze Monster. Use Freeze Monster 50 times.

Here boy!
Use the Distract Monster Special from your inventory 50 times
-Brigand and Gentleman can purchase Distract Monster. Use Distract Monster 50 times.

Doc
Use a first-aid kit 50 times
-Preacher/Scientist start with First-Aid, Mercenary can get First-Aid but that will take a long while (Near the end of World 3 in single player). Use First-Aid 50 times.

Learning Curve
Die 100 times
-In single player, only you that counts for this achievement. Two players, I do not know. Get killed 100 times.

No “I” in Team
Resurrect 10x dead characters in co-op games
-This can be done on either mode you must have the partner remove from the level (0 life left). During the Bonus screen you must say yes to resurrect your partner. Resurrect 10 times.

Root of all Evil
Collect 50000 money
-Grab as much money you can find. This will take many play through to get 50000 money.

These achievements are best done from the beginning to the end. You may use the old savegame (continue button), there is no using the password to work around it and it will take 2 hours per playthrough. I would put Six Shooter achievement here but this one is a little peculiar.


First One is the Hardest
Kill your first enemy
-The very first achievement you get. Just shoot or crash at an enemy.

Cleared The Forest
Clear world 1, The Forest.
-Finish World 1.

Cleared The Workshops
Clear world 2, The Workshops
-Finish World 2.

Cleared Fortesque Mansion
Clear world 3, The Fortesque Mansion
-Finish World 3.

Chaos
Destroy the Chaos Engine
-Beat the game/boss from World 4 on single player only.

Double Chaos
Destroy the Chaos Engine with 2 players alive
-Beat the game/boss from World 4 on two players mode only regardless if your partner is dead. You can still can get this achievement in "solo two players mode" simply pick your best mercenary (mine is Navvie) and pick whatever for the second player. Once your in the game, use the second controller to suicide that player untill it has zero life and whenever you end a level do not resurrect that player. From there after, your money is concentrated to the first player unlike in singleplayer mode where it is split into half.

Bullet Dodger
Kill 20 enemies in a single run without losing any health
-It is best done at the beginning of the World 1 where your enemies are moving and shooting slow. You must not get yourself hurt and it must be you who killed the enemies.

Bounty Hunter
Collect 10000 money in a single play through
-In single player if you have found every secrets as possible, by the time you reach World 4 level 3 this achievement will pop up. In two player mode if you are soloing it "you get to keep all the money".

Locksmith
Find 28 gold keys in a single run
-If you have played like me (finding all secrets) you will get an achievement pop as soon you get a gold key in World 4 level 1. There is a lot gold keys in World 3 level 1 but this will require you to skip some good loot such as 2 Weapon Power Up, 3 Special Power and 200 Money.

Hard Nailed
Complete the campaign in a single run
-Play from the beginning to the end (defeat The Chaos Engine) without using password. I am not sure if using the continue button would void this achievement.

Upgrade achievements can only be done at the end of all level 2 and 4 (except World 4 level 4) during Equipe Character screen. It is highly recommended that you do not purchase weapon power up, special power or extra life as you can find them through out the game.

Once you reach World 4 level 2, you must maximize : Skill, Health, Speed, Weapon Power Up and purchase all secondary Special Ability.

There is a bug regarding to these achievements. Say you are at World 3 level 4 you bought everything except health because you ran out of money you start playing World 4 level 1 and 2 with great skills or with near the end of a level 2 bad luck you died and come back with full health. Once you enter the exit of level 2 and jump into Equipe Character screen, you won't be able to get the achievement as everything now is maximize. You must be spending money on upgrade in order to work but to be safe make sure your health is not 100%.


Supremacy
Fully upgrade a single statistic for any character
-Can be done with any character. It is always the Skill that get upgraded first.

Selfish
Fully upgrade the Brigand character


Refined
Fully upgrade the Gentleman character


Ransacked
Fully upgrade the Thug character


Maxed-out
Fully upgrade the Mercenary character


Ultimate Weapon
Fully upgrade the Navvie character


Transcendence
Fully upgrade the Scientist character
-There is another bug for the Preacher/Scientist if your weapon shoot like the picture below during next Equipe Character screen there will be 3 Weapon Power Up left if you purchase them the next time you go shopping it will be there again so don't waste your money.

There is only two achievements that will give you no description how to achieve it.


Pwned!
Reach Game Over with less than 1000 score
-If your as Badass as I am you keep this achievement for last.

Perfect Day
24 hours played in total
-This achievement seem to be bug however I believe I have the answer for that. You must play the game for 24 hours without pausing the game or be in the game menu. The hours you play is cumulative so it is safe to quit the game and play another day. I got this achievement at my 27th hours of total game play. If you guys or gals who does not want to play this game but still want this achievement just simply start a game and go idle.

Six Shooter will test your dedication and put you into misery as you may have to replay the same character again from the beginning to the end of the Earth and spent 14 hours without exiting the game.


Six Shooter
Complete single-player campaign playing as all six characters
-Six Shooter can only be obtain in single player with all 6 characters played that will take at least 12 hours of game play.


Here goes my story of getting this achievement :

I previous finish Brigand, Preacher/Scientist, Mercenary, Gentleman and Navvie in that order in one go (from start to the end without losing all my life, no pun intended).
I turn on the game, playing Thug, the hardest character to play, finish the game in one go but nothing happen not a single pop up.
I replayed Gentleman than Brigand than Mercenary in one go, still nothing happen.
I replayed Preacher/Scientist one of the best to beat the Chaos Engine, finish in one go without losing a life single life in the final level it finally pop up as soon you see the Chaos Engine explode.

Now I do not know why it took so long to pop up I did not use any password to skip all the levels (I normally don't take notes) nor did I use an old savegame to continued. Took me 14 hours without exiting to desktop to get it.

Let me know if this guide has been helpfull or needs to be improve. Leave a comment and I shall see what I can do. You may request me to make a new guide. Please do rate this guide. Thank you.