Ruiner

You will find the flamethrower periodically over the course of the game but if you want to grind this achievement, the best place to do is the Walkways of Terror level. It's the fourth to last level (including the prologue) and during it you will meet the new enemy type: Drained Hosts. They have low health and come in swarms, either spawning from Dead Host Piles (see Ashes to Ashes achievement) or climbing out of pits. From the start of the level, after you meet the escaped Host, you will find a flamethrower. Equip it and move forward and you will find your first Dead Host Pile. It will spawn a single Drained Host at a time so you can sit there and torch them as they come but it will take a little bit of time. Preferably, burn the Dead Host Pile and move forward until they start to climb up onto the walkway. If you just go a little further they will climb up in front of and behind you, usually 2 in front, 1 in back (though sometimes one side might stop for a moment). As far as I know, they have no limit to spawning so you can torch through all of them to get a bunch of kills. There is another flamethrower you passed to get to this point so you can refill ammo there. If you have the last Grid Converter skill, you can use it at full health to refill your ammo, letting you burn away at will. It should not take long to get 420 kills this way.

If you have any questions or comments, please let me know. I'll try to edit the solution if anything is unclear.

Solution above is fine, but you can work on this after you reach level 10 (9 works but 10 is better) you will then be able to unlock SUPPLY DROP. Hold and select the hellfire
weapon. After every battle use and move out of it's way. Pick up the weapon and now your cooking with gasoline every battle. You can transfer points at any time.


If you go to the location in the solution above I suggest using the skill GHOST BREAK before you burn them with . This will help get you to 50 hacked enemies in the achievement mind control master. This is also a great place to get shield bash kills and possibly your 3 killed with 1 explosion.

While doing this you can mash the to get some ruiner kills. I found every so often it would pop up quickly, but if I wasn't mashing the button I wouldn't be able to respond fast enough before they died.