Speaking generally, she took us more work to get down in 10man than she should have. (She was our fifth heroic boss downed.) We had two fundamental strat issues that we didn't have right and that made it incredibly difficult. (More about those issues at the bottom.) Of course along the way we did a lot of things right. So while I have only been killing her for three weeks I have a lot of practice with the hunter pieces.
Hunter Duties in the Boss-Tank Phase
In this phase you should be a normal ranged DPS. You still have to think. Spread out for better beam coverage. Coordinate and do a beam dance with another ranged. Get out at 2 stacks. You'll likely pick up 1-3 more as you're trying to escape. Get back in immediately when your stacks drop. If you're targeted by all the beams, move to minimum range of the boss so others can more easily cover beams targeting you. If you're targeted for an Ice Tomb, move to the assigned position for tombs. This is normally not right behind Hagara. It is often to the side. You want melee to easily free you and still dps Hagara. But Ice Tombs create a line-of-sight block and you don't want to make healers move 30 yards just to heal the tank.
Hunter Duties in the Frost Phase
When the frost phase starts you should flip on Aspect of the Pack. It's also nice if you have the Glyph of Aspect of the Pack which extends the pack by 15 yards. Pack aspect will not cause stuns in this phase. Your group will love you.
You also need to do two things with your pet. First, you need to call it back to you. I have the default setup and hit control-2. If you don't then your pet will die from watery entrenchment while attacking Hagara. You also don't want it going back to attacking Hagara after downing a binding crystal. The second thing you want to do with your pet is to have it always attacking a binding crystal even if you can't. Your pet will not be affected by the ice wall and can survive cutting through the middle zone of watery entrenchment. The pet just can't stay in there.
Generally we will have melee start on one binding crystal and have range on the next one out, but we keep everyone together. We move as a group and have people affected by the frost flake stacking slow debuff dip into the middle area. Once inside we cleanse them. If they are cleansed while they have the watery entrenchment debuff that you get for stepping in the middle then no slow trap area is created when they're cleansed. If you have the stacking slowing debuff and it's not being cleansed and you have entrenchment, then you can cleanse it yourself with Master's Call. You can do this for others if you're handy with Master's Call. Generally, you want to leave this to healers.
You want to always be DPS'ng crystals. Use kiting angles to move and fire. You are the leader and make sure that the rest of the raid keeps up with you. You'll want to stop and fire steadies or cobras. Make your movement clear. Melee tends to want to stay on a crystal too long. Yell at them.
Various times you may find it useful to hang back. If you only have one crystal or if too many people have fallen behind. It is fine to dip into the middle to dodge the ice wall. You just don't want to do it too often. You can also disengage through the ice wall. This is pretty dicey and unforgiving though. Neither your start or end position can be in the wall or you'll take the massive damage. You also have to factor in the latency of your connection.
After the frost phase ends you really really want to flip out of Aspect of the Pack. I hit my DPS power-up while she's stunned with the 100% damage bonus. This includes a flip to Aspect of the Hawk. Here it is:
/use Kiroptyric Sigil
/cast !Aspect of the Hawk
/cast [target=pettarget, exists] Rabid
/cast Roar of Courage
/cast Furious Howl
Hunter Duties in the Lightning Phase in 10 Man Heroic
The lightning phase is boring if you're not chaining lightning. Chain lightning!
When the phase starts you need to do some pet micro-management. You do not want your pet by your side or anywhere near the elemental or charging conductor. You also want to flip to Aspect of the Wild to lessen the damage from the lightning. Attacking Hagara will not cause her any damage but it also will not cause any damage to your pet. It's a good way to get the pet out of the way. I was surprised that I could reduce it all to one macro (which I have some crazy lightning as the icon):
/petpassiveThis switches your pet to passive so the pet doesn't attack your target or attack any target that is attacking you. It also has the pet get out of the way by attacking Hagara. And of course flipping on some protection from lightning. It does work in only one click.
/cast !Aspect of the Wild
Click the macro and DPS the elemental. Make sure not to be in the path of the chaining in case one of the storm pillar AoE [missiles] comes down on you. You'll need to run that path. After the storm pillar lands move into position to chain the lightning. You and your teammate keep a good spacing and chain half of it and keep going until you meet the other team. You can use deterrence for some meager damage reduction. It doesn't affect the chaining. Your pet can chain the lightning as well, so you really want it out of the way. It's not an efficient way to speeding anything up. It will just add damage to you.
After the chaining is done and Hagara is free to be attacked, reset your pet and your aspect. I do this via my DPS power-up macro (same as above).
Fundamental Strat Issues in 10 Man Heroic
Our first fundamental strat issue was that we originally never tanked her towards the edge with the only thing on her front 180 degrees being the tank. (With the beams to her rear of course.) We had tanked her in the middle. We do the beam partner dance okay but stacks of 5 and an Ice Lances will often kill.
Our second fundamental strat issue was how we linked the lightning. In heroic 10man there are EIGHT crystal conductors. In normal 10, normal 25, LFR, and heroic 25 there are only FOUR. So there weren't many strats for this that we could look up. You do have to run a lot to chain it. What we had been doing was using four teams of two linking the lightning. We'd pre-position two groups at the left and right back second set of conductors and have two groups chain the first four conductors. Then the second teams kick in. The theory being they'd take too much damage if it was the same. Well it turns out that it's a lot easier if you just have two groups of two work clockwise and counter-clockwise respectively. Healing can be more concentrated on those four and if you want, you can use three healers and let them focus on healing instead of chaining. There are many tricks here (blink, sprint, heroic leap, disengage), but you don't need tricks to get it done very quickly.
Our third issue -- though nothing we did wrong in particular -- was that we would often have different tanks with different abilities. During Hagara's focused assault it is nice to GTFO of it. In heroic you can't just back up. So with our warrior tank, she'd have a heroic leap, intervene, and defensive CDs. Our feral would generally just have the defensive CDs. Our DK would have a huge blood shield and defensive CDs. The warrior had lots of options, but re-positioning after was an issue for the ice lances. Additionally, we'd have help from the raid. Beyond the normal bubbles and what-not, we'd have a priest pull them out of it and have a holy paladin taunt. Spam heals in 10mans just don't guaranty the tank will live. So far it seems like the DK tank is the most easy to keep up. Raid support is just gravy.
You can tell which phase (Lightning or Frost) she will send you to by the magic on her axes during her normal tank phase. They'll either crackle with lightning or a cool frost.