In order to complete this quest, you need to have multiple people using their Pulsing Life Crystal. (obtained from Valithria Dreamwalker for the random raid weekly) Having multiple people using the quest item, will make the buff Life Infusion stack faster. You CAN have one person do it, but it will go faster the more people that do it. You need to get 30 stacks on her in 10-player, and 75 stacks for the 25-player encounter. She will do an emote when she hits 20% life, and only after that are you allowed to use the item. You have 10 seconds for someone in the raid to use the Pulsing Life Crystal before the entire stack of buffs falls off. If your stacks of Mystic Buffet start to get high, you can hide behind a frost pillar until they drop. But be careful, as you can have the entire stack drop if your raid isn't careful about managing it. Sindragosa gains the buff Soul Preservation once the correct number of stacks are attained.
Once you complete the quest, you need to head to the top of Wyrmrest Temple in Dragonblight to turn it in.