Herald of Xotli: Combat

Starting Out
What you'll first want to take into account when choosing a combo/hellfire rotation is what your stat numbers are at. If you're loaded with stamina, but lacking mana (less than 125% mana over stamina) you will want to focus on using roughly 2-3 combos for every hellfire breath. If the opposite is true for stats, as it probably will be, use Hellfire breath whenever it is available. From there you will want to make sure you debuff the target with whatever is available or at least necessary for the target: stick with the Molten Steel Slash +damage taken debuff and occasional Burn to Death for single, non-elite/boss mobs, go all out on debuffs for anything more difficult (Inferno Curse, Fiery Enervation, etc.) to maximize both your outgoing dps and your tanks incoming damage in the case of Fiery Enervation.

Mana and Stamina Control
Having self control when it comes to mana and stamina is absolutely key to surviving drawn out battles in the early levels. Once you obtain the Body and Spirit Wrack combo and have a point or two in mana steal for it, be sure to use it whenever you drop below about 75% mana and it is available for use. When using this paired with full Feats in +Spirit Wrack you will receive about 200% mana over stamina. Instant Demonic Fortitude feat should be used similarly, when you're running low on stamina as it will yield about 10% of your total stamina for about 6% of your total mana. The damage caused by Spirit Wrack won't hinder your dps too much and given the fact that Demonic Fortitude can be instant, neither of these sources of energy return will have much of an effect on your overall damage so there's nothing to worry about there.
For those seeking a different route, speccing into the General Feat tree will yield a hefty amount of mana regen and return, but no stamina. So choosing to avoid Spirit Wrack as a whole isn't all that bad of a decision, granted you will probably still use the combo on magic users simply for the mana it drains. Points can also be put into certain Possession traits for regeneration, although both Avatar and General are more effective.

Surviving
First and foremost decide on what you need to survive, then see IIIc. for possible Feat specs.

PvP: Focusing on your hit points and damage reduction are key, along with a secondary focus on quick and hard area of effect damage to kill them faster then they kill you. For this you will need to spend points on all 3 trees, with about 50% of your points into Possession and the rest in Avatar, til Mindless brute at least, and General, 3rd tier abilities are key here and the 4th tier will help as well. From there, abilities such as Improved Word of Command and Roots of Hell will both be huge survivability boosts in PVP, either removing knockdowns for a period of time or taking melee classes out of the picture entirely.

PvE: Put skills into Subtlety first of all, it can save you some feat points depending on what you do next. Now you can choose to put some focus on the General Tree for the -5% Hate Modifier and Misdirection, replacing Demonic Leech and Ether Thirst with their counterparts in the General tree, and removing excess points from another place. Or you could focus entirely on handling your damage, from what I have seen thus far, giving your tank a couple seconds to gain some hate will allow you to go at about 80% for the remainder of the fight, most likely more if you focus on using Molten Steel Slash to drop some of your hate. Note that we have yet to do any big raids or anything at this point so large group hate is hard to foresee as of now, although reducing you hate anyway you can, whether it be gear, control, skills or feats,will help you stay alive longer in the long run and possibly offset the loss of DPS it may cause.