It's all about the timing and teamwork, also make sure the sword holder is a level 31. (Beaten him on all three of my characters, without a g-horn twice) The number one problem I've seen is death. You waste to much time when someone dies, but you can recover if you know what you are doing. If you need help I will be on at 3 eastern :) Fastest time I've seen is below, perhaps it will help you.
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I've literally been trying to finish Crota all weekend with different pickup groups, would greatly appreciate some help today before the reset if you don't mind. PSN OVchron I'll be on around 5ish eastern standard
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[spoiler] Destiny - RAID - Crota Final Fight in 3 minutes -…: http://youtu.be/dSo-BQFpA4c[ /spoiler]