Due to slightly steep DPS requirements, consider bringing six healers for 25-man version of this fight. However, I suspect three healers would be enough to do the job on 10-man.
Right as you start the encounter, you'll want to the raid to stay as close together as possible. Alysrazor will drop a large nuke on the entire raid called Firestorm. It won't exactly be fatal, but you'll want to heal up the raid as fast as possible before dealing with the various druids. Once everyone has healed up and headed to their assigned locations, phase 1 should kick in.
This is the most stressful part. The different druids who are shooting out fireballs can be dodged. There are lava worms that breathe out fire while rotating (Lava Spew). These worms can spawn at any time, and the only way to get rid of them is for the tanks to bring their charges close by. Be alert when tanks are bringing their hatchlings in, because they may get affected by Lava Spew.
Steer clear of the middle, if you can. Alysrazor likes to cut through, straight down, and just cleave everything in her path. I've been known to get singed multiple times while going from one side to the other. I put myself on tank healing, and sometimes I need to chase down the tank. Yet there is so much stuff in my way, I get tunnel vision and don't see that large fire hawk impaling me. If you need to, ask for someone to call out when they see Alysrazor taking a dive. At least you'll be able to move to a better spot right away until she lifts off again.
Important: If the hatchlings that are on the tanks start throwing a Tantrum, it falls to you to keep the tanks alive until the hatchlings get brought to a worm. If they're under the effects of Gushing Wound, let their health drop to 50% until the debuff wears off. Keep an instant heal on standby, and hit it when the debuff isn't affecting the tanks anymore.
Feel free to use mana cooldowns here, as this part of the encounter can be draining (especially if players aren't doing their part to dodge attacks).
Guilds that have just started this may have a hard time getting the hatchlings down before phase 2 arrives.
If a hatchling is still alive when phase 2 comes, don't worry. Your raid still has a few seconds to kill it before the phase kicks into high gear. It is possible to heal a tank who is tanking both a hatchling and dodging tornadoes. It isn't always easy -- but it is possible. Be very liberal and vocal with your healing here.
I've found many players struggle with this phase. The first few times can be disorienting. During the transition from phase 1 to phase 2, make sure you place yourself both away from the center of the area (Firey Vortex) and away from the edge (Harsh Winds). If you're anywhere near them, you will get melted and die a horrible death.
I learned that the hard way. Learn from my mistakes!
The best way to get around this is to chase a tornado until another tornado from the opposite direction comes by. Switch to that lane and then chase that one down. Keep repeating this. You'll be running around in a circular fashion. Don't worry if you don't get it right the first few times. It takes a bit of practice getting into position and then timing lane switches and movement.
This is by far the easiest part of the encounter. Simply stand behind Alysrazor and DPS away. Every time you DPS her, you'll get mana back. She emits Essence of the Green; whenever she is hit by a spell, 10% of the caster's maximum mana is promptly restored. Don't hold back in this phase!
Once phase 4 kicks in, raid members should still be relatively close to each other. In fact, everyone should still be stacked up from the last phase. Once Alysrazor reaches 50 Molten Power, she'll start using an ability called Blazing Buffet that's about 10,000 damage per second! Start cycling through raid cooldowns and single-target cooldowns for your tanks. This phase will last about 25 seconds. If you can survive that, you're back to phase 1 again!
All in all, most guilds will go through about two full cycles before taking her down. If everyone is still alive, the kill should occur right around the second time your raid gets to phase 3 or phase 4.
Need advice on working with the healers in your guild? Raid Rx has you covered. Send your questions about raid healing to firstname.lastname@example.org. For less healer-centric raiding advice, visit Ready Check for advanced tactics and advice for the endgame raider.