He took (or is trying to take) the light from us, that's what the whole story of D2 is about. This is something that should influence the mechanics of the encounter in which we take him down, be that in the vanilla Raid, or in an expansion. For example, he could use Guardian abilities against them during the fight. Perhaps his Royal Guard could use the subclasses we used to have in Destiny 1 (Centurians with Radiance spamming grenades, Psions with Bladedancer, and Phalanxes with Ward of Dawn), while Gary himself could somehow switch elements mid-fight. Finally, I think that the enemies (or at least Gary) should be able to Respawn during the fight. Think about it, that's one of the things that make guardians unique in the Destiny universe and what makes them so effective at what they do. Imagine fighting an enemy that can do the same thing.