Loken used to be one of my least favourite heroic bosses in WOTLK, right up there with Ley-Guardian Eregos of Oculus infamy. Even though I ran heroics as soon as I hit eighty on my first character, I was one of the last people in my guild to get the Timely Death achievement. Somehow this fight just turned every Halls of Lightning run into a nightmare. To this day I've never seen the "proper" strategy of running out of the lightning nova seen executed efficiently. Eventually I'd always be pressured into just healing through the damage as everyone stayed in (because "it's much easier that way" - yeah right, for everyone but me), people would die left and right, and only with some luck the tank and I would eventually finish the guy off after kiting him around the room for five minutes.
Lately however I've managed to overcome my fear of Halls of Lightning in general and Loken in specific. Today the instance was a "double daily" on my server and I ended up healing through it three times, once on my priest, once on my paladin and once on my druid. We never wiped, but it was certainly insightful.
On my priest, the main thing that struck me was what an immense difference good gear makes on this fight. I remember when we did this with the whole party wearing mostly blue-quality items, the dps doing maybe one thousand damage per second each (if we were lucky) and me being expected to heal through three, four, five lightning novas without running out of mana (which I never really managed). These days, in Ulduar gear, everybody does two or three times that much damage, so Loken dies much more quickly, and my heals also hit for a ton more. I never had trouble topping everyone off after each nova, even though we never tried to avoid it.
The lesson I learned on my paladin was that the class of your healer doesn't really matter on this fight. I've noticed that holy paladins have a tendency to moan about their lack of a big aoe healing spell as if it was something hugely crippling. I've always had my doubts about this, considering that whenever we raid it's a holy paladin who tops the healing meters on nearly every fight. The experimental instance runs and Naxx raids that I did as holy myself only confirmed my suspicions that the class's aoe healing is really powerful anyway. Sure, every now and then there was an occasion where I thought that having something like circle of healing would be handy, but that's no different than me sometimes wishing that my priest had more heal-over-time spells or that my druid had prayer of mending. It's situational. Overall I did fine.
Especially in small group content like a five-man instance, beacon of light and the glyph of holy light are extremely powerful group healing tools. In the end I actually found healing the Loken fight on my paladin much easier than on any of my other characters, because you don't even have to do much thinking. On my priest I have to decide whether to cast prayer of healing first, or prayer of mending, or put a renew on myself first... whatever. On my paladin I just have to make sure that beacon of light stays on the tank and then I spam holy light bombs on whoever is lowest on health until the boss is dead. It's a no-brainer and it works, because if I get my self-buffs up at the start, a holy light only takes as long to cast as one of my priest's flash heals, while healing for much more than a priesty greater heal would do in a hundred years. Paladins lacking group healing power? Bah I say.
Finally on my druid, the lesson that got driven home was that gear really matters on this fight. Wait, didn't I say that already? Let's change it slightly to stamina matters. My druid's healing gear is actually pretty nice for someone who hit eighty only quite recently, but several pieces of it are cloth and thus low on stamina. We only managed to one-shot Loken due to the paladin tank's skill and burning of cooldowns, as I actually died on a nova early on. With buffs my little tree has about 17k health. That's fine for surviving a nova of 14.5k damage. It's not fine when you immediately get hit by arc lightning right afterwards for 2.5k damage. Splat. In hindsight I suspect that this was the reason for a lot of the wipes I experienced early on when both healer and dps only had about 16k health max. People would always go "oh, that's fine, his nova only hits for 14k" without taking into consideration any damage that could follow immediately afterwards. I don't think you can consider yourself safe to survive the nova with less than 18k health really, only if you get lucky with whom he targets with his random attacks.
In summary: Any healing class has the tools to heal through the aoe damage of the lightning nova. However, good gear makes a huge difference in the difficulty of this endeavour, and a healthy amount of stamina is pretty much required - I'd recommend at least 18k health buffed for everyone. Also, if anyone ever figures out a way to make a whole party of people run out of the nova properly, let me know. I'm still kind of intrigued by the idea.
14 minutes ago