Breaking him can only be done when staggering him in his enraged form. He enrages at ~85% HP, but keep in mind he disappears at 66%. So while staggering, you cannot do too much damage either. Bible helps for this. For my blue GL, unequipping all items except Sidereal Weapon (+0) and ulting brought him to just over 85%. From there, expertise nodes + pet + VPH meant I could stagger him in one rotation. Then throw Corro and destruction spells. After I figured out the right rotations, one run took ~30s (depending on his spawn loc).

If you also need the 50 kill achievement you can equiq Orb of Sacred Nature (from paradise) to guarantee a one-shot wiht your T spell.