This one requires you to "catch fire from every possible enemy and extinguish it in the water." There are 9 enemies that can set Lara on fire: 3 men with flamethrowers in the DIVING AREA, 5 men with flamethrowers in THE DECK, plus the game's final boss (SPOILER!). Since you'll burn through a lot of medipacks getting past them, you may want to save before each encounter, catch fire and extinguish it for the achievement, then reload and replay that section without catching fire. It should still count toward the achievement. The Steam game tracks your progress, although there is sometimes substantial lag before the counter updates.
To the best of my knowledge, you do not have to do this in a single playthrough. So if you missed any of the earlier flamethrower encounters, you can skip around in the game, loading various earlier saves to get it done. Using a downloaded save will reset the achievement. However, using cheat codes is absolutely fine.
The main walkthrough covers every flamethrower encounter. But if you prefer, you can tackle this achievement during a separate playthrough, using the cheat for all-weapons (which also gives medipacks) and the level-skip cheat. (Roadmap follows.)
If you've already completed some of the encounters and just need to clean up, follow these links:
TO UNLOCK "FIRE SAFETY" IN ONE PLAYTHROUGH USING CHEATS:
First, load a save from before encountering the first flamethrower. (If you like, you can download one of mine.) Or, start a new game and level-skip to the beginning of the DIVING AREA. Play through to the BLUE CARD DOOR normally. Then, if necessary use the all-weapons cheat for max health packs.
DIVING AREA - FIRST FLAMETHROWER (1/9)

The first flamethrower is just beyond the BLUE CARD DOOR. Unlock it, enter the SQUARE HALLWAY with Uzis ready. Turn right...

...and take running jumps along the wide hallway.

Kill the first 2 dogs, just around the first corner.

Roll or turn around and kill 2 more dogs.

Roll and run/jump counterclockwise to the far side of the square hallway. (You'll pass a hatch with a wheel. Just ignore it for now.)

When you reach the second hatch with a wheel...

...open it quickly before the flamethrower approaches. (He should come from the right, following in Lara's footsteps.)

Run through, down the stairs, and into the ROOM WITH THE POOL AND CIRCULAR SAW.

Roll and kill the bat man.

Then wait at the bottom of the stairs for the flamethrower to approach. Let him set Lara on fire. (You may have to move partway up the stairs for the flames to reach you.) Then quickly roll...

...run, and jump into the pool, using medipacks as you go in order to survive. (If you're playing on PC, the keyboard hotkeys for small and large medipacks are 0 and 9, respectively.) Climb out of the water and finish off the flamethrower.
Now play through the level normally to collect the FIRST MACHINE CHIP and return to the ROOM WITH THE POOL AND CIRCULAR SAW. (Obviously, you can use your most powerful weapons and skip unnecessary pickups. If you run low on supplies, just use the cheat code again.)
DIVING AREA - SECOND FLAMETHROWER (2/9)

Use the FIRST MACHINE CHIP to open the door in the ROOM WITH THE POOL AND CIRCULAR SAW.

Equip the Uzis or automatics. Move down the ramp, not quite to the bottom. There are 2 enemies in the the next ROOM WITH POOL AND ORANGE TANKS: A guy with a club on the far side of the pool and one with a flamethrower lurking just inside on the left. Start shooting the club guy as soon as Lara locks on.

When he falls, move all the way into the room...

...let the flamethrower set Lara on fire, then immediately run off the edge into the pool.

Swim to the far corner of the pool, climb out of the water...

...and finish off the flamethrower before he can toast you again.
DIVING AREA - THIRD FLAMETHROWER (3/9)

Continue playing through the level normally, skipping unnecessary pickups, until you reach the entrance to the CENTRAL ROOM WITH THE RED LOCK (beneath the HELIPAD).

Ready the shotgun or Uzis, step off the edge, and fall into the room below.

The flamethrower is behind the central structure. Ignore him and run to the left. Kill the enemy with the baton using the shotgun or Uzis.

Then run past him and through the door opposite the RED LOCK. This door opens automatically as you approach.

Run straight across the hall and start down the stairs leading down to the ROOM WITH THE POOL AND CIRCULAR SAW.

Stop halfway down, turn around, and wait for the flamethrower.

When he reaches the top of the stairs, roll...

...let him set Lara on fire, then continue down the stairs, healing as needed.

Veer right and fall into the pool.
Climb out of the water and use the level-skip cheat to move on to 40 FATHOMS. Since you begin that level underwater, you'll need to swim past the sharks and into the shipwreck until you reach the CARGO HOLD WITH FLOATING CRATES (walkthrough). Climb out of the water there and level-skip again.
WRECK OF THE MARIA DORIA begins in a short passageway. You should have enough room to level-skip again.
LIVING QUARTERS also starts underwater, so you'll need to open the trapdoor and climb into the wreck (walkthrough) before you can level-skip a fourth time to reach THE DECK.
The "Fire Safety" guide continues on the next page.
DARK MODE: I plan to add optional dark mode for these pages, but my current priority is updating the walkthroughs. Meanwhile, may I suggest a reputable dark mode browser extension, such as Dark Reader? (Download only from trusted sites, like darkreader.org or your browser's official add-on/extension site.)
Walkthrough text and diagrams copyright © Stellalune (email ). Feel free to copy or print this walkthrough for personal use. By all means, share it with friends, but please include this credit line so people can send me their feedback. No part of this walkthrough may be reproduced on another site without permission.