If your MTing Twintania, You still always want to have a HUGE HP Pool Just in case something prevents you from putting up your Mitigation in time. (Pacification from Berserk for example, or having used unchained at the wrong moment.) Twintania will still smack you for 7800 if you don't manage to toss out that IB at the right time whle Vengence is on CD.
If your an OT for Twintania, the 3 Dragons are very weak, so you don't need all that MT mitigation and HP to deal with them. Defiance alone does the trick, and toss out a Vengence at the start to mitigate from 3 of them. After that, it's full on DPS to burn them down quickly. Then it's on to...... You guessed, it being 2nd on Aggro, and Dealing as much DPS as possible to Twintania in order to end the forsaken fight. At that point, ALL of your mitigation skills are borderline worthless, as you won't be taking threat on Twintania, unless your group failed. if you mention Snakes now, the Big Snake is for the MT, the 4 little ones are easy, and I have accidentally tanked them perfectly fine in Sword Oath when I was forced to play my undergeared Paladin. (Which means I was playing as a 6.3K HP Flash bot with no mitigation aside from Shield Blocks)