Each time your raid brings XT down 25%, his heart will drop and take 50% more damage. Destroying his heart will make him flip out bring him into hard mode. No matter if you destroy it the first time it drops, or the last he will automatically refill up on health! However, there will no longer be adds that spawn. Once you unlock hard mode the Light Bombs will spawn Life Sparks that deal significant damage to those in range of it. You need to have your ranged DPS take these down as quickly as possible! The Gravity Bombs will also now spawn void zones. It helps to have designated areas for your raid to drop these. This makes it easy to spot where the Life Sparks will spawn and also keep the Void Zones out of everyone's way.