I Was Saving That For Later

There are little webbed mobs around the room. To break one of them free you need to kill one of the small Caustic Spiderling adds on top of the webs, which will cause them to be hit by Caustic Eruption. Once freed they're a friendly mob that will run around. When the boss casts Gossamer Burst she will pull these in for a quick little mid-combat snack. Once you free and have her eat all 5 you can kill the boss. Keep in mind that when she reaches the last floor at the top of her room she no longer casts Gossamer Burst so you must break all 5 before she gets up there.

As for where they are, I've included some screenshots below with world markers at t heir locations.

Web 1 - Bottom floor, on your right when you enter the room in the corner. Blue marker in the first screenshot.
Web 2 - Bottom floor, on your left by the base of the stairs. Green marker in the first screenshot.
Web 3 - Second floor, left hand wall at the top of the stairs. Purple in the second screenshot.
Web 4 - Second floor, around the corner to the right before the staircase to the third floor. Red in the second screenshot.
Web 5 - Third floor, left hand wall between the two staircases. Yellow in the third screenshot.

First floor webs
Second floor webs
Third floor screenshot

I took the last screenshot from the final floor to provide a better view of the place in the room it is but forgot to remove orange marker. There is no web on the final floor.

Also Web 4 is very dangerously close to the edge so I'd recommend you send a DK out there to grip the add and kill it, that way if a Gossamer Burst happens while you're out there you can Death's Advance it to avoid being pulled off the edge. Alternatively you can just wait until after one goes off before going out to that ledge.

If you have any questions about the achievements feel free to add me to Btag or Discord!

Btag - FrostyShot#1667
Discord - Yarini#9999

Easy Solo for Level 80-100 Worldsoul Saga & beyond...

1. Normal Difficulty.

1b. Achievement tracker criteria will turn from red to white when criteria met.

2. Begin fight.

3. Notice the webbed captive to your right, run over there and kill the little 'Caustic Spiderling' on top of it, the tiny webbed captive will run away frantic.

4. Proceed to free the next four webbed captives the same way, another one on the left side against the wall, then on the second floor against another wall, and another towards the front of the boss to the extreme right of the ramp going up, and then third floor another against the back wall, be careful not to be at the front edge for Gossamer Burst or you will get pulled over.

5. With all 5 captives freed and having been pulled away by Gossamer Burst, achievement criteria has been met.

6. Kill Sennarth.

7. ?

8. Profit. Achievement Completed.

Video with all the boss kills for the Glory of the Vault Raider: https://www.youtube.com/watch?v=GSXY8qf-nq8&t=40s

Instructions for Sennarth - I Was Saving That For Later: You need to free five mysterious captives from their cocoons.

-1st floor: you’ll find the first cocoon on your right when you enter the room and another one is to the left next to the stairs,
-2nd floor: one is on the left side in front of the stairs when you come up, and the other is on the right very close to the edge next to the boss.
-3rd floor: The last one is just as you come up the stairs.
https://www.youtube.com/watch?v=GSXY8qf-nq8&t=40s gives a walkthrough of where the captives are.

To free the captives, you need to kill the caustic spider adds next to the cocoons, causing them to be hit by a caustic eruption, which is the small green swirly thing on the floor, freeing the captives. All of these captives will eventually meet their fate when Sennarth pulls them in with its web. Once all five captives have been freed and have met their inevitable doom, just kill the boss as normal.