Since the boss has a buff that makes him take 75% less damage, that's 75% less threat that you do, so threat is really dicey. Healing threat is unavoidable. DPS need to not attack the boss until a ways into the phase (potentially after the first tentacle is down).
I've picked him up going into Phase 2 on my PT tank in 16man HM and we definitely had some threat issues to work on when we were first learning the transition.
Proximity matters a lot going into that phase. Make sure DPS wait at the entrance until the first tank is in position and don't DPS the boss. The tank should be the first out the gate to establish initial aggro, with necessary healers behind.
We swap Guard to healers while we're getting sucked into the portal to curb their threat a bit in the phase.
Once the transition starts, I usually hit him with 2 globals of ranged abilities as I'm transitioning to the platform upon which I tank, blow my AOE taunt, and then keep attacking until the 6seconds of taunt is up. Then I use my single target taunt. When you taunt, it forces the boss to attack you for 6sec and gives you 130% of the highest threat target (even if it's already attacking you). I work my taunts into my regular rotation on that fight (keeping one up only for Scream transitions).
Once you're all settled on a platform and have a good thread lead, DPS can attack (but they should be busy with Anomalies and tentacles).
use single target taunt first. its basic as you want your taung off CD quicker and as Single taunt has low cd, it should be used first.
Dont make it a 1 tank fight, Swap taunts with the other tank. I would think you single taunting, then other aoe taunting, then you aoe taunting should keep the boss on you till scream, at which point other tank has his single taunt ready and from then on it should be sweet.
also as someone said, use force pull from first platform. high threat 0 damage.