From Blue Post:
Here is a strategy I have seen used a lot on heroic Loken with a paladin healing.
* Everyone moves to melee together. The paladin casts Beacon on himself.
* When you get the emote for Lightning Nova, everyone moves back together.
* There is often a misperception that Lightning Nova has a huge range. It doesn't. You don't have to move far.
* You also don't need to run back to Loken. Let him come to you. Then next Lightning Nova, move backwards again. You can do a slow kite all the way around the globe if you need to.
* Of Loken's three abilities, Lightning Nova is the worst, followed by Pulsing Shockwave. Arc Lightning can do a lot of damage, but it's not always worth spreading out to try and avoid it because then the other two abilities can nail you. However, if you are getting multiple stacks of Arc Lightning, the damage can add up. If it gets to be a problem, try to spread out slightly but still move together as a group.
* This isn't a healing fight at the end of the day. It's a movement and coordination fight, much like Murmur in Shadow Labs.
Loken does kill more players than any other boss. He's not a pushover. Good luck!
At the end of the day, it does come down to running out for lightning nova and stacking up around him the rest of the time.