Before the pull
If you are Horde
You need a Nature Resistance gear for this fight : 150-170 NR should be enough.
You also need a frost damage wand, such as Mana Channelling Wand
You'll need 3-4 Greater Nature Resistance potions for this fight, pop one before the pull.
Look up the name of the shaman in your group: you'll need to be close to him for the poison cleansing totem.
Put a shield on yourself before the pull.
If you are Ally
If you have a decent gear and decent health pool, you don't need NR. If it's not the case, you better put a NR set. But you do need a frost damage wand and NR pots.
Pop a NR pot and put a shield on yourself before the pull.
Phase one
This phase is when Viscidus is in the middle of the room and not frozen yet.
If you are Horde.
Go with your shaman and wand the boss. Refresh your NR pot when it needs to be.
Do not heal, unless the raid is very low life. Your job is to freeze the boss, the shamans are here to heal.
At the end of the fight, the raid may be low life and you can throw some PoH and renews on the raid.
If you are Ally
Your job is still to wand the boss to freeze him up.
But the lack of shamans will make it hard for paladins and druid to both heal and cleanse. So you'll have to do the healing.
If you are in a caster group, throw a poh at each poison bolt. Start casting your poh 2s before the bolt, so it goes off just after the damages.
Otherwise, just keep wanding and throw some renews here and there.
Refresh your NR pot when it needs to be.
Phase two
This phase is when Viscidus is in the middle of the room, frozen.
If the raid only needs few heals, just go melee and throw some low ranks renews on the people that need it while meleeing.
Phase three
When the boss disappears, go out of the middle of the room (go back with your shaman if horde).
Heal up the people that need to before the end of the phase.
Put a shield on yourself before the boss comes back.
Use fade before the boss reappears.