For your information:
ELEMENTAL RUIN:
Tempest of Set (Curse of Set), Herald of Xotli (Inferno Curse)
ELEMENTAL WRACK:
Tempest of Set (Lightning Sparks), Demonologist (Chaotic Blast), Herald of Xotli (Molten Steel Slash)
So you pretty much rely on ToS and Demo to do debuffing job then if you don't.
Not entirely true. Damage is made up from various bonuses, but the baseline is:
DPS = Weapon DPS + Combat Rating Bonus
So if you have 135dps from the Trial of Flame, and use Glorious Exultation of Xotli, thats gonna add 121,5 dps and bring you to 256,5 dps on your weapon for 10 seconds. Mind you, that is during finisher of Burn to Death, and then measly increasing the dot for a few seconds, and affecting the white hits you do for the next combo. And then, it makes -20% for 15 seconds (?) which will drop your weapon damage with 27 dps down to 108 dps. Most likely this will affect a few Pillar of Infernal Flame, the Fire Lance...
Flame Cleave adds 710 combat rating. Thats a permanent increase with zero drawbacks. 710 combat rating equals 19,4 dps. Hell on Earth has 90 seconds cooldown and add 4075 combat rating equals 111,3 dps. Zero drawbacks, instant cast, and last longer than Exultation (though with a longer recast). 19,4 dps + 111,3 dps = 130,7 dps. 130,7 dps + 135 dps = 265,7 dps for 15 seconds and no drawbacks.
All these numbers are added together *before* the combo damage is calculated. So it is just plain wrong to say that "combo damage for hox scales badly". For some reason the Exultation increases Burn to Death dot by 10-12% but adding combat rating fire does not.
Here is the complete formula if you want to read it (thanks cins).
Combo Damage = [StarterDPS x Starter hit animation time] + [DPS x white hit animation time] (repeat this for every combo step along the way) + [FinisherDPS x Finisher animation time]