Welcome back! With the Spider Quarter, Construct Quarter, Plague Quarter, and Death Knight Quarter out of your way, the teleport to Sapphiron's room (and from there, Kel'Thuzad) will become available. There's no trash to worry about, just two big, ugly bosses between you and the completion of Naxxramas. Tonight we'll deal with Sapphiron; tomorrow we'll head in to finish off the big lich himself.
A lot of people consider Sapphiron to be more difficult than the Wrath version of Kel'Thuzad, and with good reason. On both the normal and heroic versions, Sapphiron functions as a test of your heal team's gear and efficiency with constant, low-level damage on the raid in addition to a nasty AoE and high tank damage. If your heal team is generally undergeared or has struggled with previous high-damage fights, you may want to shelve Sapphiron for the time being (especially if you're doing the 25-man version and most of the raid doesn't have frost resist).
Boss abilities (Phase 1)
In this phase, Sapphiron stays firmly planted on the ground while beating the living snot out of your tank. Raid members should stay relatively close to each other to assist with the efficiency of AoE healing abilities while keeping a firm eye on where Chill is going. On normal, you can safely keep your raid on either side of Sapphiron. On heroic, while there are arguments both for and against splitting the raid, I tend to favor splitting as long as players have the sense not to send all of the decursers and healers to a single side. Having half the raid on either side of Sapphiron minimizes the potential effects of Chill and creates fewer clusters are players move in Phase 2.
- Melee: Sapphiron hits hard. Expect to see melee blows in the area of 9-12K.
- Cleave and Tail-Swipe: Standard dragon abilities: melee DPS should stay around his hind legs.
- Frost Aura: A constant AoE frost damage aura that ticks each second. On the 25-man version especially, the damage is sufficiently potent that frost resist gear is advised (at least until your healers are well-geared). On the 10-man version, you probably won't need it unless you're having issues learning the fight, but it will make healing a lot easier. Because the entire raid needs to be healed constantly and Sapphiron is a relatively long fight, the heal team is well-advised to use high-mp5 gear.
- Life Drain: A curse randomly applied to 5 players on heroic and 2 players on normal every 24 seconds. Should be decursed immediately it heals Sapphiron even as it deals damage to players.
- Chill: A Blizzard-like effect that moves slowly around the room for the duration of the fight. Sapphiron can spawn more than one at a time, and your melee DPS are particularly vulnerable to getting wiped out by this if they're not careful. If you're caught in this, movement is slowed by 50% for 10 seconds, which is particularly dangerous during Phase 2.
Sapphiron takes to the air after 45 seconds and casts Ice Bolts while preparing to do a Frost Breath. Raid members need to stay somewhat spread out on this phase. If you're caught within 5 yards of someone who takes an Ice Bolt to the face, you'll take a potentially lethal amount of damage.
- Ice Bolt: This will be targeted at 2 players on normal and 3 players on heroic. Ice Bolt is randomly cast and does a little Frost damage (easily healable) while also encasing the target in a block of ice. The raid needs to be aware of Ice Bolted targets, as those iceblocks will come in handy while Sapphiron charges up --
- Frost Breath: An AoE frost attack instantly lethal to anyone who's not huddled behind an Ice Bolted player's iceblock.
The reason for A is that Ice Bolt targets are entirely random. On the 10-man version, this is unlikely to be an issue as most of the raid should stay on one side of Sapphiron anyway, but it's a much bigger problem on 25-man. While it's tempting to keep the raid split during Phase 2 in order to avoid extra splash damage from Ice Bolt or a badly-placed Chill, you run the very real risk that all three Ice Bolts wlil go to one half of the raid, thus dooming the other half.
After this happened to us once, we changed our strategy and moved the raid to the back of Sapphiron's room (on and around the steps to Kel'Thuzad's room) during Phase 2 while raid members continue to maintain a 5-10 yard distance from each other. This minimizes the potential difficulties wrought by the RNG.
The reason for B is that, while It's very tempting to get to a "safe" area as quickly as possible, a collection of players behind an ice block while Sapphiron still has 1 or 2 more targets to Ice Bolt can wreck your raid. If Ice Bolt goes to a player behind an ice block with others in close proximity, everyone concerned will take a massive amount of frost damage. Because Frost Breath is a 7-second cast, you should have ample time to reach an ice block without hiding behind the first that occurs. Wait for Sapphiron to do 2 Ice Bolts on normal (3 on heroic), then move. After Frost Breath, Sapphiron will land and Phase 1 will recommence.
So, Phase 2 ideally goes as follows: Sapphiron takes off, raid runs to the back of the room avoiding Chill (or stays where it is, as you prefer), waits for the Ice Bolts, runs behind the ice blocks, then resumes Phase 1 positions as Sapphiron lands.
Lather, rinse, repeat -- that's the fight.
Tank: Only one tank is required for Sapphiron. If you can get away with frost resist gear while remaining defense-capped, wear it; if you can't, don't bother. Tank him where he "spawns" as you begin the fight, don't move him (doing so will usually get your melee DPS tail-swiped), and keep him facing the entrance to the room at all times (some raids prefer him to be tanked facing the steps to Kel'Thuzad's room). He'll need to be picked up in the transition from Phase 2 to Phase 1, and you need to be very careful to move him as little as possible. Move straight back; strafing here will usually get the raid tail-swiped or cleaved. Most tanks favor high-mitigation and/or high-stamina gear on this fight, as part of your heal team may be occupied at any given moment avoiding Chill.
DPS: This is not a particularly melee-friendly fight as Sapphiron's range for both cleave and tail-swipe are very large. You'll need to be in close quarters around his hind leg, and if Chill hits melee, the damage is tremendous. All DPS has to be aware of Chill and its likely path at all times. Otherwise, the only other thing you'll really need to worry about is getting well out of range while the tank picks Sapphiron up between phase 2 and 1. If you have to hug the walls, do it. Pump out as much damage as you can otherwise; the constant healing needed for Frost Aura is essentially a soft enrage.
Healers: The fight depends on you and the raid's ability to avoid taking unnecessary damage. HoT classes are very useful for raid healing as Frost Aura damage is constant, but you'll also need to worry about a lot of tank damage. Druids and Shamans on 25-man should make sure they're not all on one side if the raid's being split; draft Mages as necessary to help decurse Life Drain. Players learning the fight frequently underestimate Chill damage and must be told that staying out of it is their responsibility. Damage from Ice Bolt should be minimal (2-3K on the targeted person) and shouldn't be a worry, but the 10K+ splash damage on any player nearby is unacceptable.
Sapphiron is the subject of the achievement The Hundred Club, which requires you to defeat him (on either normal and heroic; achievement's the same on both) without any raid member having a frost resist value of more than 100. As Wowhead commenters point out, a frost-specced Death Knight with 3/3 Acclimation can eighty-six any effort to get this as they're virtually guaranteed to get more than 100 frost resistance at some point during the fight. Things like Frost Resistance Aura or the Frost Resistance Totem will also do it, so players have to be very aware of what talents or abilities they've got at their disposal that will have any effect on the achievement. Healing is substantially more difficult with a low frost resist value on the raid (more so on 25-man) but not impossible, especially once your heal team is geared.
10-MAN vs. 25 MAN
Most of the differences have already been described above, but the usual applies: more damage and health on Sapphiron on the heroic version, more people cursed at a time, and one more Ice Bolt on heroic.