Gears of War 2

You can easily get this during the campaign. You'll first get the Scorcher right before turning on the power in the facility in Act 3. From then on, you'll face only Wretches for a long period of time. Simply use the Scorcher on every Wretch you see, they'll go down incredibly quickly on Normal difficulty. You should find plenty of ammo as long as you explore the side rooms a bit, and remember that a perfect active reload on the Scorcher gives you a longer flame, so you can start hitting the Wretches from farther away.

If need be, after you shut down the AI, you'll fight a bunch of Sires. They go down very quickly as well and rush you like Wretches.

Easiest to do in "Horde" mode. After the first couple of waves, one of the new enemies will be "Flame Boomers", they carry the "Scorcher Flamethrower", So once you kill take their weapon. I would suggest you only use it on wretches and drones because they die fastest and you don't have a lot of fuel for the flamethrower. Every wave there will be at least one "Flame Boomer" so just keep picking up the weapon off their dead bodies and repeat.

this one is very easy i did it by playing annex with bots
i made every weapon a flametrower and burnt all enemies
and if you die you'll respawn in 5 sec and then you can pick up a new flametrowher and start again and then ding achievement you can use this method for other weapons

This is a relatively easy achievement to get if you are going for the full 1750. With the Dark Corners maps there was an achievement titled "Is It Hot or Is That Just You?" which is for scorching 25 blood mounts. If you are going for this (Best with friends so they can pick you up) you will get this after about 13 of those kills considering you typically scorch the rider before you finish off the Bloodmount.

Also there is the scorcher in Act 3 as Codster mentioned with all of the wretches which will net you this achievement quickly if you aren't too concerned about the blood mounts and want to get this achievement as quick as possible.

These kills can be done in any game type from multiplayer to horde or campaign so chances are you will end up getting at least 30 kills with this weapon by the time you are near the end. Especially with 2 campaign playthroughs and the amount of horde you have to do!

You can easily do this in a private execution or warzone match with a second controller, just change the weapon load outs to scorcher, select whatever map you feel like playing on (I did avalanche) go pick it up and run to your enemy’s spawn and scorch them, repeat 30 times and the achievement is yours

Start ACT 3 from the beginning (chapter 1), follow the mission until you find the flamethrower on the ground. Get it, turn on the lights and go back to a wing with several Wretchers, burn them down and reset the checkpoint until you get 30/30.