really whatever happened to not ummm exploiting the fact that a held hydra drops the sheilds. I did the trial a while back with my main (38 at the time) and the mission wasn't impossibly hard to do the right way. Sure it took awhile, and sure we had some tense moments, but hey these days I doubt I'll ever see another team that wants to do the sewers with out holding down the hydra.
Also is it just me or does this method, ie holding the head, not only require a type of character but eliminate nearly any reason to ever take a scrapper incapable of hardcore tanking with you.
One more final comment, why not a dark armor scrapper. I know you mentioned Regen, but a DA scrapper is one of the only classes currently capable of maxing their Psychic Resistance (or very nearly maxing).
Dispersion Bubble?