I tried this; the barrels burst into flames if I am seen (I used 4 per your suggestion)
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Ok thanks. I got it; what I did was put a heavy weight on the pressure plate and then used invisibility to do the rest.
You don't need anything special other than 4 empty barrels (or anything of similar weight) which are available near where you fought the guardian, if you didn't destroy them.
On the western side, stay hidden in the smoke cloud and put 4 empty barrels over the lava vent. This will prevent lava from flowing through even if detected. Stand on the pressure plate next to it to deactivate the 1st lava trap on the eastern side. Go there with another character, sneak behind the sentinel and pull the lever to open the western gate. Pull the western lever to open the eastern gate.
