Full common mitigation lets you tank and self-heal a captain-general. It may be a bit excessive, unless that's what you see yourself doing, or if you are sick of being killed by huge heal-aggro. If freeps are the main problem, they use mostly Light and Beleriand (at least the good ones are). You can push "other" mitigations up just over 50% IIRC (Beleriand always 2% lower), but of course that makes you worse against Fire (light) or Westernesse (Beleriand). Since the standard is around 30%, it may not look worthwhile.
Disclaimer: this is by no means a partularly popular or even the best build; but personally I have:
Corruptions: Power for Damage 2, Health for Damage 2, Common mit 1, Common mit 2, Tactical res 2
(not sure if tactical is really what I want, I was going on an old guide to stuns/mezzes - have to review that now after Bk14)
Class: Snap Out of it, Empowering, Enhanced Bubble, Enhanced Rez, Harsh language (+25% heal)
It's a full healing spec... I am there only to heal heal heal, rez and bubble, and heal. And I don't notice myself dieing any more than other WLs. I think if you're in the RAT your survival depends on either a bubble (or 2), or correlates directly with how many other WLs are in the raid. Maybe Diminishing Returns will change that, and I'll notice it soon enough.
I have only about 6000 health with IP, and 1070 power. I still run out of power occasionally, but only if things are going crazy in a throne room and I have to tank as well as group-heal. On those occasions I usually run out of health also.
If you really want to solo a keep, you can spec out of the Enhanced rez, bubble and Snap, and put in Block, Avoidance and Damage. It will still take you an hour at least though
edit: trying out 49% Beleriand mitigation for a while, see how it fits