My regular group ran the Crota raid about a dozen times and we couldn't finish the big guy off. We learned something each time, though and eventually figured it out. Crota was down to about 1/3 health. Everyone was running out of heavy. This was pre-Xur selling Icebreaker of heavy synth. We're all firing away with P&Ts and a couple of Icebreakers...he finally kneels and my teammate takes him out. Watching Crota's health bar drop to nothing was one of the most rewarding things I've experienced in gaming.