While the size of your Combustion is largely dependant on your raid's approach to Halfus, you do have some degree of control over how to maximize the damage it can do in any given strategy:
The first debate posed forward, and one that will significantly reduce the randomness of your ignites, is usage of Fireball or Scorch in the seconds building up to the usage of Combustion:
* Use of Fireball will give you a slightly larger ignite, whilst opening yourself to the risk of having your ignite fall off.
* Scorch will still yield a large ignite, with a higher chance of pulling off a great combustion, at the expense of not achieving your theoretical best Combustion.
Although neither approach will give you a certainty of getting the Combustion of your dreams, based on the cast time & travel time of Fireball compared to that of Ignite the argument leans towards Ignite being used as the filler nuke. However, one is always free to 'take the risk' with Fireball in the hopes of achieving a higher damage output, bearing in mind that there will be an inherent higher risk of losing your ignite.
Again there is still a chance of simply having a lack of crits with Scorch, just that there is less of a chance of it within the given time window compared to Fireball due to the lack of a travel time and shorter base cast on Scorch.
Delaying Flame Orb usage until after Combustion has set sail should also yield better results.
I hadn't seen of hard-casting pyroblasts, other than the single pre-pull. I'm surprised this didn't yield satisfactory results as it is more likely to satisfy two out of the three Combustion requirements (and the two RNG-based ones at that). A quick log browse from the top parses show both Ignite and Fireball fillers before the Combustion. Could this be elaborated on? It seems counter-intuitive that our hardest-hitting spell isn't yielding superior results to the two alternatives given the circumstances.
I also agree with Tyrian that mastery stacking will yield excellent results. Assuming a four drakes released at the start approach the same can be argued for haste effects on pull, to allow for more Scorch/Fireball while building up the ignite. These are slightly harder to control and outside of your direct control, both falling under the category of preparing outside of combat. Also included are the obvious consumable/buff stacking associated with any min/maxing: T10 set bonus, volcanic potion, flame cap, dark intent. Are there any other pre-fight improvements to be added? During ICC the
Might of Malygos buff from Dragonblight could be obtained to achieve extreme numbers during Blood Queen, and would not disappear after zone-in. This could possibly be added as it will still be effective, if it has not been fixed.
Fel-Infused buff from Desolace has also been mentioned.
Lastly, once the Combustion is away your most effective approach should be to Blast wave, and use Impact procs every time they come up to spread your damage increased DoTs on the drakes. This cleave damage should yield superior results than single target damage on Heroic given a four drake strategy, as well as contributing to bringing Halfus himself down. Do not delay Impact spreading for a Hot Streak to proc, as just the ignite itself ticking away on all targets will make it worthwhile. Flamestrike wouldn't benefit from the increased damage on Halfus, and as such wouldn't be worth using compared to the above other than to 'force' Impact procs once a large ignite is reached if one isn't up already. The same can be argued for Blast Wave. Speccing into Improved Fire Blast would be worthwhile as well, to get more chances of a crit Fire Blast out of the GCD you are using anyway.