RAGE

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You can get all 3 of these weapons on while still on the first disc, the Sentry Bot and Sentry Turret are far better than the RC Car so use them.

Easiest way to do this is while playing Mutant Bash TV, you can play this mini game as many times as you want until you get to the second disc.
There are plenty of mutants when you play this and they are easily killed.

Edit: As said by "Mr PhyziX" in one of the comments, you can still return to the first disc.

Set the Sentry Turret up in a corner and it will be able to hit just about everything and nothing will be able to sneak up on it.

For the Sentry Bot, just drop it anywhere and it can easily hold it's own against the mutants.

But Wait... There's More!!

Like this you can easily get the 3 other achievements for getting however many kills with one Sentry Turret, Sentry Bot or RC Car while doing this event so its an easy 140 Gamerscore.

Holy S***... There's Still More!!!

Did I mention you are also making a lot of money while doing Mutant Bash TV!! You can even get the achievement for getting $750 in one Mutant Bash TV run. I got it each time so it shouldn't to be too hard.

If you have the Rage Sewer Missions DLC (that came with new copies of the game), this achievement is a cake walk. Simply go to the first 3 sewers and let one of those spider bots get all the kills. The sewers are located by the Ark, the Outrigger Settlement, and right outside Wellsprings. The spider bots will not die in these sewers for some reason, so you can get all the kills with them. This should give you almost 100 kills alone.

If you don't have that DLC, perhaps try playing through a rerun episode of Mutant Bash TV with spider bots (don't worry about getting par time). Or just buy a bunch of parts from a store and make a bunch of turrets and bots and use them throughout the game.

This can be earned easily throughout the campaign by simply utilizing them in combat but one of the best areas to rack up a large amount of kills is in the Abandoned Distillery missions on the second disc. In both missions you'll have to hold off two large waves of mutants that come charging at you from the other end of an open area. Simply place two sentry turrets (preferably the advanced version) and let them go to work. Completing all four waves will land you upwards of 60 or so kills and the rest will easily come through continued play.

I found the best place was where you pick up the TNT at blue line station. Once you pick up the TNT, save your game and get ready to put down sentry turrets/ sentry bots or rc cars but i would avoid these.
once you get a few kills with these and have run out of enemys or equipment then just load and rinse and repeat. This works for all of the kill achievements.
Make sure that whichever you are getting kills with you stick with until you get the achievement. i recommend the sentry turret. Hope this helps

i got this in blue line station just build a lot of the advanced sentry turrets and have two out at all times i made it all the way through and didn't have to kill anything. if that doesn't make the achievement pop for you then you should only need a few more kills.

I actually did this by restarting and redoing mutant tv bash (re-run) on easy and it still seemed to count towards the achievement. I wasn't trying for the achievement, I was just messing around with different weapons etc.

I was using a combination of rc car, bot and turret, although I think the bot is the best

I just kept playing mutant bash tv, Use sentry bots in all of the rooms except the jungle room with the spinning monkey as they just get smashed, Instead stand in the corner where the exit gats is and place a turret, just kill any that get behind it, I also placed 2 sentry bots each time i used them,

I had to play through 4 times to get all 100.

I also managed to win the achievment for winning 750 dollars during this.

all you have to do is let the turrets and bots do the work, Hit one mutant with a well placed wingstick and let the robots do the rest, Youll get 100% accuracy and ge your £750 in no time

I'm going to side with KrunchbiteRAWR and xGunna 94x. Blue Line station, right where you pick up the TNT. However I'll differ from them in one thing. I prefer using the sentry bots rather than the turrets, as the bots can melee the mutants and they even jump down below to engage the muties before they start to climb. Another part of the map you can use them, is in the station concourse area (the big, open area, where the giant mutant will pop up).