Numbers are low, all the healers are under performing. I can't say much about dps as i dont play one and i know shit when it comes to single target / aoe but it appears you have 3 ok dps and maybe 1 ok healer.
Learn better rotation, and better concentration + focus during the fight. They are't playing very fast with numbers like those
Then again, you guys are only 12 manning it, so i have no idea what logs would look like at that stage.
but i mean a lot of the fight is spreading from Horns of Valor, Soaking for Light Beam, and then using a BIG heal cool down after adds.
Are you guys failing during the runes? or just too many people dieing over time. The mechanics are all really telegraphed and it's more of an endurance fight than anything else.
Here are my guilds logs for 20 man heroic
https://www.warcraftlogs.com/reports/wbJLD1TmcWMtFGRj