Don't forget goodies behind left cauldron.
Don't walk on the pink part of the floor (wasps).
Pull down levers #1, #2, #3 and #4. At "5 = 2", pull up lever #2. To get lever #6, simple jump action from 1 step back, then monkey swing. Lever #7 opens the secret door, below the bridge where you encountered the first 2 guardians. It's timed. Save the game in front of the (unpulled) lever #7. The most difficult part is getting the floor.
The "regular" solution goes through the water pool (Theresa), and there is an alternative (Stella) using previous plates. But if you have half a medipack to waste, try the following:
Look at right/down pic: immediately after the block near #7, turn 90 degrees left and jump to the plate near #6, hop back action, drop to the floor [about 50% damage], roll, run to exit (jumping the wasp triggerning pink section), go, go, go...
Don't loose time with the cauldrons: just sprint and be lucky. At the end of the "2 guardians" bridge, jump down at right. Another hint: Lara would like to pre-open the door at the end of the ladder (do it in a first exploration); if you choose this way, it's safer to return in front of lever #7 via the crawlspace (that is: don't use the ladder down way).