Blizzard awards achievements to a guild based on there being at least 8 players with a kill. We decided to use this as a baseline and then allow for a little more wiggle room. We currently look for 7 guild members with a boss kill before awarding them credit, but we may or may not adjust this number in the future based on user feedback. Our goal is to make our rankings as accurate as possible and as true to being a “guild” raid as possible.
Note: These requirements are still in place for cross-faction. You must still meet the above requirements for a kill to count as a guild kill.