This is a rather annoying encounter that can be hard depending on your raid

Mar'li Abilities:

Baby Spiders: Mar'li will summon 4 spiders at the beginning of the fight and then randomly summon more later on. These spiders grow quickly and after 15-30 seconds can be dealing a lot of damage. Take them down fast.

Web: When Mar'li goes into spider form she will drop down a web rooting those affected and will go after anyone randomly because of this.

Silence: Mar'li will have a AoE silence effect when she enters spider form silencing all for 5
seconds.

Poison: A AoE poison DoT that deals 400-600 damage per second. This should be dispelled immediately as this can kill off your healers.

Drain Life: Mar'li will drain life on raid members and it regenerates her life extremely fast. Make sure to dispel this as fast as possible.

Mar'li Fight:

To fight Mar'li requires at least 2 tanks 1 for Mar'li and 1 for the baby spiders. When she goes in to Spider form and webs people the second tank will need to taunt her and bring her to the MT again.

The First 4 spiders that Mar'li summons need to be taken out by mages with frost nova + AoE. The first 10% should be done so that everyone is ready for her spider form and does not have to be a burn her down phase.

When she goes into spider form have your 2nd tank make sure he is out of range of the web and can move in and taunt her and bring her back to the main-tank.

After this it's all about repeating you strategy each time she swtiches forms. Also when she is down at low hp around 25-35% she will attempt to drain life someone which needs to be dispelled as fast as possible as if she gets to much life back she can kill you.

Overall the main annoyance of this fight is the additional spiders that Mar'li will summon and if you can master taking those out she will be a easy fight.