I would not recommend doing this. I'm pretty sure it is a damage-loss not spamming 100% crit <1sec casttime pyroblasts into the boss for nearly 30 seconds just to allow your ICDs to come back.

It's a good tip to make a Fire-Spec just for this fight since you won't need to spec fire anywhere else. You can skip most AoE-Talents but Pyromanic is still good. Even if 10% more haste won't affect your Pyro-Casts it will affect your living bomb and combustion. I also took "Invocation" in my Alysrazor Build giving me some extra damage in the burnout-phase.

On Trinkets:

I used DMC:V and Soul Casket (costs 1650 Justice Points) for this fight. Soul Casket is really nice for this fight though it is only ilvl359 it still has a good amount of mastery and the on-use cooldown is 2min meaning it can be used with/before combustion everytime.

If i had a necromantic focus i would probably go with Soul Casket + Necromantic Focus. DMC is still a good option too because of mastery and the short/good proccing ICD.

Rune of Zeth is nice too but its critrating is wasted in this fight even when reforging to mastery you loose 168 mastery rating and the 1277 Intellect are inferior to Soul Caskets' 1926 Spellpower in this fight. Generally the Rune may be the better trinket for Fire, but not in this fight. Since DMC:V can be bought in the auction house (but every mage should have one anyways) and Soul Casket can be bought with justice points there isn't much reason to use Rune of Zeth.

I was never lucky enough to get a "Theralions Mirror" so i could not test it on alysrazor. But i have one on my alt and from that experience i would say that the mirror isn't really viable in this fight simply because of the high ICD and the bad proc-chance. In theory the trinket seems perfect for this fight but it all doesn't matter if it won't proc when you need it (and i'm pretty sure it will happen a lot of times).

Images for 2t12 seems to proc ignite, I'll have to watch this closely and maybe issue a fix for combustionhelper.

The entire point of alys is getting the best possible combustion on her expose phase possible. Using your cooldowns correctly and not "on cooldown" is always a smart decision.

This is partly accurate, but misleading...

In order for your Combustion to gain benefit from Pyromaniac, Pyromaniac must already be up. If Pyromaniac is up, this means you used Impact. If your Impact is now down, you spend more time fishing for Impact, which means more time wasted on the icds you need to use.

If you Impact immediately(with 2 death grips or a controlled InvoCS), then get a strong combustion to impact, it certainly would be better to do so. I'd be interested to see the math on 25 Stacks to 10% with Pyromaniac up, giving 3(?) or 4 extra Combustion ticks -

This would mean practically speaking, you are trying to get an Impact, Impacting, trying to get another Impact, then hoping for a Crit Ignite with Procs, then Impacting.

Soul Casket is just an awful decision all around. If the argument is -
1. You can use 1926 Spellpower with your Combuston, i say I would rather have 1277 Intellect with my Combustion.
2. If the argument is 168 Passive Mastery, thats not a terrible line of reasoning, but your net gain of 1277(Roz)+1600(DMC:V)+580(LWE)= 3457 Intellect(Nearly double your total Intellect) is hilariously good.
3. Crit rating is not wasted for this fight - For a large portion of the fight you dont have 25 stacks, and having effective uptime on your Ignites is the name of the game anyway.(I.e. - Not having Crit Ignites is wasting the vast majority of any mastery you create for yourself anyway.)

2 Million DPS - Fire Mage - Heroic Alysrazor - YouTube (If you havent seen the 2 Million DPS peak :P)

Is there some extra benefit I'm missing (besides high ranking on WoL) to spreading your combustion to the adds during the burn phase on Alysrazor?

I've noticed a strange problem yesterday. While before my 3 week absence a
Blastwave + flamestrike into 4-5 mobs was virtually guaranteed to give an impact proc.

I noticed yesterday, that I barely got impact procs at all. Has the behavior changed recently?
I looked through the blue-tracker but couldn't find anything..

edit: typos

Ive been noticing the exact same behavior over the last couple of weeks. I couldnt find any documented changes, but it does not seem to be a guaranteed impact proc anymore.

The main problem is, that with combustion/impact not planable, fire looses a significant amount of AE power.

For Alysrazor, I have been favoring the Glyph of Frostfire and Glyph of Living Bomb. The third glyph is usually Molten Armor because you won't always have the 24 stacks of the buff to ensure auto crits, and Molten Armor benefits both Pyroblast and Fireball, in addition to Living Bomb, Flame Orb, and Frostfire Bolt. Once you get to 24 stacks, the benefit Fireball has over Frostfire Bolt is essentially eclipsed because of 100% crit chance, and the up-time of Frostfire Bolt's DoT (assuming you are vigilant with it's maintenance once hardcast Pyroblast becomes the better nuke) definitely makes a difference. It adds damage itself, and also adds to Combustion's damage.

This started as a pvp bug with reliance double dipping with ignite when combustion is being applied. I've noticed a 25-30% decrease in combustion damage in PvP after this hot fix. Has anyone else noticed any Pve ramifications?

Unless they've fixed things, FFB's combustion effects are way less than they should be - FFB ticks for ~300 were giving combustion ticks for ~19 instead of the expected ~100. This should probably be retested now that they've changed other combustion mechanics though.

Of course, if you're crit capped with the buff so the other glyphs aren't useful, FFB may well still be better.

You misunderstand, I am in no way saying that FFB can ever replace Pyroblast spam with that many stacks, just that the extra dot is a good addition to overall DPS in an evnviroment where you have guaranteed crits and massive amounts of haste. I only cast FFB to refresh the dot after getting enough stacks to switch to hardcasting Pyroblast.