Space Hulk: Tactics

There a probably a few ways to get this, especially using the flamer.

What I did was find an explosive cannister on the map, and put my marine in the square where it is. I then saved my game, and got my Marine to melee attack the explosive. It blew up, and my Marine survived, with the achievement then unlocking at this point.

By saving just prior, if your marine dies, you can just reload the checkpoint, and try again.

Good luck!!

Guaranteed way to get this.

The Apothecary has a card upgrade that grants immunity to fog & flames to the targeted terminator. You will also need a heavy weapons bearer with the flamer equipped.

Obviously it's a bit random as to when you will be dealt this card, although the faster you use them the more likely it will be to show up. Once it does, save your game and play the card on your flamer terminator (or another one within range) & then lay down the heat.

Reload once the achievement pops & continue on as you were.

The other solutions mentioned are probably more reliable, but it is possible to get this playing as the genestealers. I got flamed by a terminator and my genestealer survive. The achievement popped.
So you should get this just playing normally if you're a bit lucky.