Before the pull
There will be 3 tanks : you need to assign healers to each of them.
You can pop a Greater Nature Protection and a Greater Arcane Protection potions before the pull. Else you risk a one shot.
The Fight
Go at your tank side and jump on the little march, this will be easier to heal from there.
There is nothing much to know about this fight, besides that the boss will MC people: dispell the sheeps on them when the MC ends and avoid getting killed by them (use magic dust, fear or iron grenade if necessary).
Don't stay in the arcane aoe damage.
Besides the tank healing, some raid member will take nature or arcane damage. Those damages can kill quickly and so you should be ready to shield and flash heal them.