Neptune's Hall - Entrance and Fire Knight
Run/jump past the undead knights...
...and step on the button with the water symbol in the MAIN ROOM of the HALL OF SEASONS to open the three nearby doors.
Go through the left door (i.e., the one with the water symbol above the lever, not the outer door).
When you pull the lever...
...the portcullises blocking the inner hallway rise. Run to the middle door...
...and save the game in a new slot before going through. The inner door opens as you approach, and once you're inside NEPTUNE'S HALL, you can't leave until you've beaten it.
The "Be Like Water" achievement/trophy requires you to finish the level without taking any damage. So plan to save in different slots and/or quick-save as you progress if things are going well.
As you enter this area, a fire knight (1) begins hurling fireballs in Lara's direction. Run forward and hide behind either of the first two pillars near the hole in the floor...
...so Lara doesn't get toasted.
Even if you're not attempting the "This is Lara!" achievement/trophy, I'd recommend sending him into the pit, just to get him out of the way. Wait behind the column and the knight should come around toward you.
Then move behind the other column near the entrance in order to lure him to that side of the hole.
Run around next to him (so he's between Lara and the hole)...
...and kick or blast him off the edge. If you can do this without taking damage, quick-save and continue. But if Lara has been injured, and you're trying for the no-damage run, reload and try again.
NOTE: I can't vouch for all versions, but the Steam game tracks your progress toward achievements like "This is Lara!" on the Achievements screen. Even after a quick-save and reload, the game should still "remember" what you did here. This will be knight 2/17 (for the entire game) if you're playing the NEPTUNE side area first.
There are 2 boxes of V-Packer shells (1-2) in the corners of the room with the big hole in the floor. If you haven't destroyed the fire knight, you can still get them. Just lure him to one of the opposite corners, run and pick up one box of ammo, and repeat.
Then proceed to the inner room and drop into the water-filled opening.
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