First of all- when accessing boss rooms through portals- DO NOT FORGET TO CLICK ON THE CHESTS IN THE ROOMS when going through them- as you might get Naga equipment from them as a additional reward. There are 2 chests before facing Timira.
When you entering the boss room, firstly you will be in the room with little "lake" and there will be bucket standing next to it. You job is to fill the bucket with the sparkles from the water (water will be sparkly every now and then, you need to be quick) and then pour it on the Shattered Waters (East side of the room). In case you get just a water in bucket- spill it on the floor. There will be mobs appearing in the waives- try to kill them as soon as they appear. Once enough amount of time is done with the bucket, you will receive a message "The water is forming into a shape! Use it to venture further!" and the chest will appear in the room- take any items from the chest and proceed to the next Portal.
Next room you will have to only click on Corrupted Trapped water (south in the room) and survive all waives of the mobs. Repeat the process until you receive message. Please note only person without number 1 next to his name can use/click on Corrupted Trapped water. Once chest appears in the room- collect items and enter through portal on east side of the room to Main room with Tamira.
Tamira the Many- Headed- there is no additional mechanisms in the main boss room. Just kill the boss. It is recommended for the EK to lure the Boss to the North-West corner of the room.
How challenging is above? Not so much. Done it on 350 RP with ED (400) and EK (400) without issues. On Higher levels can be done solo.
Recommended Video and explanations from Tibia Wiki https://tibia.fandom.com/wiki/Within_the_Tides_Quest
Video by Lee Kun https://www.youtube.com/watch?v=uPQ139DGXAk&t=1s