During the encounter, when you are standing inside the large green circle, you get a buff that increases all damage 50% and healing 50%, but you also get a debuff called Overcharged, increasing damage taken 5% every few seconds, stacking up. The only way to get rid of the debuff is to leave the green circle.
While fighting the boss i found that you can stand on the very edge of the green circle, just inside the phasing zone, and instead of running out and running back in, you can simply jump and it will reset the negative stacks but keep you in the 50% damage/healing buff.