1 tank dk + 3 dps dks + 1 resto druid here, I never spread out for Ionar and have always just healed through the damage. My dks are all different specs to cover all the buffs but I do have 3 glyphed rune taps (makes so many bosses alot easier to deal with)...remember to make use of your AMZs and AMSs (frost presence + IBF as an AMS backup) when the damage is overwhelming. Druid != Priest, but in my case I almost always need to innervate on this boss as I'm spam casting hots/wg for a bulk of the time and my druid is geared out pretty decently