That's you're opinion, which isn't quite accurate. The solar damage cause damage over time and takes down his shield just as fast. I'm a warlock, personally I use ghorn, but I'll throw a grenade at crota too for the damage over time effect and it sticks. Ive beaten the raid with guardians using the truth, Hezen of vengeance, hunger of crota, and dragons breath and their wasn't an issue at all. His shield still dropped just as fast. Gjallarhorn is great but not as much of a necessity as people make it out to be for that raid.