Borderlands

In order to unlock this achievement you will need to kill 25 enemies with a corrosive weapon. If you are wondering if the weapon you have does corrosive damage just go to your inventory screen and in the area that shows the damage, accuracy, etc. of the weapon you will see a green haz mat sign just like the one in the achievement pic as well as a multiplier of x1, x2, or x4 which indicates the chance that the effect will proc. You can also do corrosive damage by shooting green barrels that are close to enemies. I've noticed 2 different green barrels, 1 that explodes and one that splashes acid. Make sure it is the 1 that you see acid splash out of. If the enemy dies as a result of this acid barrel it will count toward this achievement as well. These barrels show up within the first 10 minutes of the game. Basically if you see green numbers coming off of the enemy you are doing corrosive damage.

I can confirm that this achievement can glitch. I had over 500 corrosive kills with no achievement. You don't need to clear your hard drive of all game information you just have to start a new character and re-do the achievement with the new character.

You can also track how many corrosive kills you have by going to the quest screen and pressing "Y" which will take you to your challenge screen. Then simply scroll down to where the corrosive kill counter is.

EDIT: MulletGuy reports that a recent patch has remedied the glitch associated with this achievement. If you met the requirements previous to the patch with no achievement, you should receive the achievement upon updating.

Weapons drop randomly through-out the game so unfortunately I wont be able to direct you to a certain in game spot to get one, I'll try and provide a few pointers though.

You can see if a weapon has an elemental damage bonus by highlighting it in your menu, where the guns stats are listed (damage, accuracy etc.) there will be a small sign indicating if it causes any elemental damage and if there is a small sign, it'll let you know what bonus damage that gun causes, in this case we're looking for a small green biohazard sign. If you find/already have a gun capable of inflicting corrosive damage, just equip it and go kill a bunch of enemies, you can track how many kills you've accumulated by going through the menus, on the quest section press Y and you'll bring up the challenges menu, look through it until you find the corrosive kills challenge and there's a counter there, that's a bit long winded so if you prefer you could always just try and keep count by writing it down or if you're doing it in sitting, keep count in your head.

There are other ways you can get this achievement, if you can't find a weapon capable of inflicting corrosive damage, try and kill enemies which are in close proximity to the green barrels scattered through-out the wastelands, you could alternatively use create a new character and use Lillith (if you're not already), at level 16 you'll be able to upgrade her melee attack and make it corrosive.

Don't forget to check the weapon vending machines too as they do stock weapons with elemental bonuses from time to time.

Finally, lots of people say the achievement is buggy and doesn't unlock when it's supposed to, while this didn't happen to me I know it has happened to a few of my friends, if this happens to you, do not continue getting kills hoping that it'll unlock!, it won't, instead back out and create a new character, it's a pain that you have to do this but it's really all you can do if you suspect that it's been bugged out for you. The bug which caused this to happen has been patched, but I've included it in the solution on the off chance it happens to you! If the achievement did bug on you, boot up your file with the new update and the achievement should pop straight away!

I've linked in a video so you can see the effect the weapon will have on an enemy if you're using a corrosive weapon.

*^{Credit to Gkthepwner for the video}

There are 4 achievements in the game tied to elemental kills, now this can be a combination of kills with weapons, grenades and barrels that are scattered around the maps.

You can tell if your weapons and grenade modes have specific elemental effects as they have an icon on them:
• Red Flame - Incendiary
• Blue Lightning Bolt - Shock
• Yellow Explosion - Explosive
• Green Radioactive Symbol - Corrosive
The barrels are also colour coded:
• Red/Orange - Incendiary
• Blue - Shock
• Yellow - Explosive
• Green - Corrosive

The easiest enemies to kill will be unshielded enemies like Skags, Rakk, Spiderants and lower level bandits, however it doesn't matter if they have a shield if your weapon is strong enough.

Get 25 kills with a combination of weapons, grenades and barrels and the achievement is yours.