Per class instructions on how to deal with the Siphon:
Druid - Melee (especially dual-wielders) stop attacking in order to prevent being hit for over 200 damage per attack from Thorns. Spellsteal (or Dispel/Purge) the Lifebloom as soon as Malacrass casts it. The Moonfire is not a big deal.
Hunter - The traps dropped are visible to the raid, so, as soon as the tank spots one, he has to kite Malacrass away from it quickly, with the melee following closely. The Freezing Trap freezes several persons (unlike the Hunter version). The snakes are easily killed with light AoE.
Mage - The Fireballs and Frostbolts are randomly targeted and can be interrupted. With good interrupts, this class drain is very easy. Since there is no additional AoE in melee range, the low damage Frost Nova is a non issue. Dispelling it is not needed.
Paladin - The Consecrate hits for 2,500 per tick and the tank and melee should move out of it very quickly. Dispel/Purge/Spellsteal Avenging Wrath as soon as it comes up: the extra damage dealt is very nasty. Interrupt Holy Light like you would with any heal.
Priest - Put Fear Ward on the MT once the Priest drain starts, CC the Mind Controlled players (the Mind Control lasts for 20 seconds) and interrupt the heals.
Rogue - Cure the Wound Poison quickly and pump up the healing because of the increased damage caused by Slice and Dice. The Blind doesn't seem to target the Malacrass tank, so it isn't a great problem. It is, however, undispellable.
Shaman - Interrupt Chain Lightning and Healing Wave. When Malacrass drops a Fire Nova Totem, kill it quickly. It does a lot of damage, but has very few hit points, so a quick focused burst from the DPS team will prevent it from firing.
Warlock - Decurse the Curse of Doom, make sure not to dispel the Unstable Affliction and spread out to minimize Rain of Fire damage. Rain of Fire ticks for 3,000 damage, so run out of it quickly.
Warrior - Save Disarm for the Warrior drain. If Malacrass does get a Mortal Strike in, constantly cast heals on the tank until the debuff wears off. Be aware of the Spell Reflect; stop casting when you see it. The Whirlwind has a short cast time, but it does hit for around 7,000 hit points on leather. It's possible to run out of it, but you need to react very quickly. Melee with slower reflexes might want to wait this phase out from a distance or bandage up after getting hit.