Video running at 2x speed to save some time. It is a mechanically uninteresting fight after all...
Really haven't been soloing much lately due to Diablo 3, not that there's much left in WOTLK that I can solo. There may still be a few bosses here and there that is possible for spriest which I haven't done, but I'm just too lazy to look for them.
Looking back now at all the WOTLK bosses I have killed solo, Sapphiron (10 man) was the first. So I decided to make it my first 25man WOTLK boss solo kill as well. So I marched in there thinking: "it's just more hp right? how hard can it be?"... Turns out it's quite a big step up.
So the 25man version poses a few challenges for the shadow priest:
1. Outheal and survive the damage. The 25man version hits for approximately 50% more damage compared to the 10man version. This will mean that a significantly higher proportion of the time will be spent on healing. From my experience, Flash Heal spamming is still the best way to go.
2. Beating the enrage timer. If the previous challenge can be overcome, then there is the issue of the enrage timer. The 25man version has three times as much health as the 10man version. This, combined with the fact that more time is spent on healing, means that the 15 minute enrage timer actually becomes a challenge to beat. But if dots are all refreshed on time, it shouldn't be too big of an issue.
3. Mana conservation. Mana shouldn't be a problem if all the mana restoring abilities (Archangel, Dispersion (glyphed), Shadowfiend and SW:D) are used on CD and replenishment up full time. However, since I specced into Inspiration for the damage mitigation, I did not have access to Archangel, thus mana regen felt some slight pressure. It still wasn't much of an issue till the end, and is definitely not as big of a challenge as the other two mentioned.
Basic strategy is to keep dots up as much as possible. Start healing when health dips below 50%. At one point the boss will begin to fly into the air. At this point you have around 10 seconds to either refresh dots or top up your health, depending on the situation. After which you will then be frozen for another 10-15 seconds but also becomes immune to damage. Although I didn't do it in the video, but it is a good idea to keep a renew up during the frozen period if necessary. After the frozen debuff goes away the boss lands and the cycle repeats.
Throughout the fight you will take additional frost aura damage as well as damage from a blizzard falling on you. I do not recommend running out of that blizzard since you'll just be running forever. Just outheal all the damage, while letting dots whittle the boss away.
Lastly keep in mind the boss will no longer have an air phase when he is near death, although I am not sure as to the exact percentage of health when this occurs.