The DPS requirements on this encounter seem to be on the high side, since you need to break the crystals during the frost phase or else your raid group will succumb to attrition. If your healers excel, you can get away with five healers, although for learning attempts, I suggest pulling in six healers instead for the extra stability and longevity.
Don't you love the intermission phases? The thrill of having healers possibly losing line of sight with the tank when Ice Tombs are up is so exciting! Luckily, you won't have to worry about that during the first phase when you start the initial pull.
Some boss mods out there are able to place icons on players that are about to get hit with an Ice Tomb. Drop shields and HoTs to buy as much time as possible for the rest of the raid to break them out of it. Compared to normal difficulty, keep in mind there's an extra tomb out there on 25-man. If you're a healer who gets stuck in a tomb, urgently signal that to the rest of your healers.
Focused Assault can catch inexperienced healers off-guard. Don't take it for granted, and load up the affected players with HoTs and mitigation abilities as needed.
Ditto for Ice Lance. Feel free to take two or three hits of them before strafing out and letting another raider take the hits. Unlike normal mode, the damage steadily increases.
I've heard methods starting with ranged players staying inside the bubble and DPSing outward to strategies involving running around the outside in multiple groups. My raid group stuck to having two groups on polar opposite sides of each other and relying on individual ability to weave in and our of Ice Wave as needed.
Dispel handling is the trickiest. Frostflake slows down players, and it needs to be removed so that they can continue staying ahead of Ice Waves. However, when dispelled, it leads a patch of frost on the ground that slows down movement speed of anyone standing on it. One wrong move, and the newly cleansed player ends up slowing down everyone near them.
We instructed affected players to run inside the Watery Entrenchment bubble during this phase and yell out for audible dispels (in the form of "Dispel," since "dispel me" could often cause confusion). Keen healers will already have their mouse cursor hovering over the afflicted player ready to dispel them once the cue is given. Doing this places the frost patch away from areas where the raid can continue to run around safely.
If you feel confident enough, you can duck inside the bubble to let a wave bypass you if you don't think you can outrace it. As a healer, I was able to strafe inside the bubble while I get my own healing spells active and ran back out when the coast was clear. The downside is that you might end up being too far away from the rest of your group to help sustain them.
Oh, and watch out for falling Icicles. As a joke, I once got Leap of Faithed into a falling block, which shot me towards another falling block and bounced me into the Ice Wave, causing instant death. We did end up having a good laugh about it afterwards.
The best method for dealing with Hagara's lightning phase is using the plus method. What we ended up doing is using six players arranged in a line from the middle of the room to the four Crystal Conductors on the outside. (If you look at it from the top down, it looks like a cross.) We kept at least one healer in each group. There's going to be one or two groups that will have an extra healer (and that's OK). Be prepared to use a raid-wide cooldown or two to buy a few additional seconds for the lightning strike to travel through the group and neutralize the crystal. Once your tank drags the elemental close to the center of the platform, it shouldn't take more than a few seconds at most to get through this phase.
If you're not sure how to array your players, I suggest keeping melee players near the middle as a start. Since they'll be adding DPS to the elemental when it's up, it makes sense for them to assume a position that's close by as opposed to forcing them to run all the way to the back of the line. Your healers can take the middle spots for the sake of healing accessibility to the group. Lastly, the ranged players can hold down the anchor at the points furthest away from the middle.
Storm Pillars aren't really much of a threat, especially if your players are spaced out appropriately.
Repeat each phase about two times and you should be able to take down this ice queen relatively easily. A typical guild can expect to spend anywhere between three to five hours.
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