We had the same issue and heard that the amount of bullets gets the black to appear in the center which is what lets the orbs damage him. So we switched to lmg's (mine should be equipped), and weve been able to stagger with 4 people up, it seriously took it from impossible to 5 tries for us. Also had anyone not on orbs keep it open while we did orbs to do max damage.

The video was our first time killing him so we were roughly in the same boat as you with no 300's! We also didn't have weapons of light. For it to work for us we had to have all 6 people alive or he wouldn't stagger. We also had several goes where people missed shots, weren't shooting or were shooting too early/late. After a few hours we stopped making silly mistakes and we nailed it. Basically just let your team know that if they're making mistakes, just be honest and admit it. That way you can solve them and get it done.