I am looking for ways to maximize Enervation. The way I cast it at the moment is
Mythic Enervation (1d6) -> Maximized (6) * Empowered (1.5). This adds up to 9 negative Levels.
Is there any way to get even more negative levels?
A maximized and empowered Enervation will cause 4+1d4/2 negative levels (mythic 6 + 1d6/2). So we are looking at something between 6 and 9 negative levels per cast.
Since enervation uses a ranged touch attack, a natural 20 on the check is a critical threat, which if confirmed, will cause double the damage. Damage here being the negative levels.
If a ray spell deals damage, you can score a critical hit just as if it were a weapon. A ray spell threatens a critical hit on a natural roll of 20 and deals double damage on a successful critical hit.
With Improved Critical, you can increase this threat range to 19-20.
Note, it is common under the rules as written that GM's do not allow enervation to cause critical hits, since it says it causes negative levels and not damage. But the rules as intended is that enervation should double the negative levels on a critical hit, as explaiend by James Jacobs (Paizo's Creative Director) regarding this same topic back in 3.5:
Yup; negative levels stack. Also... you can crit someone with enervation, inflicting 2d4 negative levels all at once with one casting of the spell. Check page 293 of the DMG for the lowdown on how energy drain and negative levels work.
So, you should consult with your GM if he allows enervation to cause critical hits or not.
Yep, a second enervation. Quicken is +4 spell levels, so this probably will take up a 8th level slot. Unless you have tricks to reduce the spell level with metamagics, which is possible, see Magical Lineage and Spell Perfection.
With magical lineage, you could add Empower Spell on this quickened enervation, to add an extra +50% damage (1d6 * 1,5 for mythic). This will also reduce your 9th level empowered and maximized enervation to a 8th level slot.
With a second enervation, causing between 1 and 9 negative levels, this should be something between 7 and 18 negative levels in a single round, not accounting for critical hits.
If both of them are critical hits, then we are looking at up to 36 negative levels.
How about four or more enervations? Cast Time Stop and you will gain 1d4+1 extra rounds to act and cast spells. If you have enough 8th and 9th level slots (which isn't impossible), you should be able to cast up to 10 enervations in a single round, or about (ridiculous, i know) 90 negative levels.
Just remember that time stop, normally, doesnt let you target other creatures while the effect lasts.
While the time stop is in effect, other creatures are invulnerable to your attacks and spells; you cannot target such creatures with any attack or spell.
However, Mythic Time Stop allows you to pick extra creatures to not be affected by your time stop and act normally. So, technically, this is still the same round, even though both you and your target acted up to 5 times.
If slots are a problem, Pearls of Power exist to fix that.
We are now in the realm of overpowered rules, be warned.
If you got access to mythic metamagic feats (which are third-party content, written by Owen K. Stephens), we have:
Mythic Empower Spell (tier 5): All variable, numeric effects of the spell are increased by +75%, including bonuses to those dice rolls.
Mythic Maximize Spell (tier 7): All variable, numeric effects of the spell are treated as 150% of their maximum.
Mythic Quicken Spell (tier 10): If a target provokes an attack of opportunity from you, you can target it with a quickened spell as your attack of opportunity.
Considering that the wording of the feats are different from the non-mythical version, I asked the author about how they are supposed to work and he confirmed that they each apply separatedly to the spell effect.
Also, as long as you have mythic power free, you can use any of these metamagics without increasing the spell level, this means that all your enervations can be maximized and empowered for a single round. By mythic tier 10, you should have at least 23 mythic power free, which allows you to do this 11 times before you run out of them.
A mythic empowered enervation deals 1d6 * 1,75 negative levels, or between 1 and 10 negative levels.
While a mythic maximized enervation deals 6 * 1,5 negative levels, or 9 negative levels.
If you do not use time stop, this means each mythically empowered and maximized mythic enervations should deal each between 10 and 19 negative levels. So we are looking at up to 38 negative levels in a single round (without critical hits or time stop).
If you mythic time stop yourself and your enemy for 5 apparent rounds, this should mean up to 190 negative levels in a single round of combat.
Other answers have addressed methods of applying vast numbers of enervations to a single target in a single round. While this certainly is one way to address the problem (and, if this is permissible, the maximum number of negative levels is limited only by the amount of in-game time your character is willing to invest), it may be unsatisfying to address the problem this way. In your question, you address the problem from the perspective of maximizing the impact of a single casting of enervation. This answer will take that approach.
First of all, we will apply all of the relevant metamagic we can, as you have already started to do:
And that's about it for metamagic, assuming you can't forward-port broken stuff from 3.5.
Next, let's take a look at mythic options:
Lastly, there's always buff spells, class abilities, and traits that increase damage rolls in a generic manner or specifically do so for spells with attack rolls. (There's too many to list here)
And you should use Limited Wish to guarantee your attack roll is automatically a success, and Greater Named Bullet (cast on an optional material component, like salt) to guarantee it's a critical threat and add up to twenty additional negative levels.
While there are a myriad of options from lowly table salt to invoking the power of the Plane of Negative Energy which can increase both your caster level and the effective spell level of enervation as well as the maximum benefits incurred from high levels, none of that actually allows you to deal additional negative levels with a casting (except by possibly allowing you to apply the Toxic spell feat, as discussed above, or by interacting with other damage boosts).
Consider the Glyph of Warding, the Greater version, used as a spell glyph. Cast a Glyph of Warding, greater on the place you are planning to have your fight, containing one empowered enervation (doing a maximum of 6 negative levels, or 12 with a critical). Set it so it can only affect evil creatures (if you are fighting evil ones), just so you and your team don't trigger it*.
You can yourself cast up to two spells per turn (there might be ways to bypass this but I'm not aware about them). One of them being quickened. You can make them both empowered (for example with Magical lineage), doing again a maximum of 12 negative levels each.
If you have access to mythic (like it seems to be the case), you can cast the mythic version, each time, so you are doing up to 6, 9 with empowered, and 18 with a critical. It makes a total of 54
For the sake of completion you can also have a Contingency of Empowered Enervation with a trigger like "when I cast Empowered Enervation", but you can't actually target someone else than you with it. Anyway, if that counts it brings you up to a maximum of 72 negative levels inflicted.
Note also that these numbers are the maximum you can get: it is actually very unlikely that all your to-hit checks are confirmed criticals and all the "damage rolls" score the maximum on the die. There are ways to improve the real expected result (like applying Maximize spell, taking feats to improve your chances to perform a critical, etc). For example if you manage to Maximize all your already empowered mythic spells they all inflict a base of 6+1d6/2 (so between 6 and 9 (or 12 and 18 with a critical hit) negative levels) instead of 1d6*1.5 (between 1 and 9 or 2 and 18) ; if you take Improved Critical(Rays) you sorta double your chances of critical hit. This last one is also doubled if you take Spell Perfection, making you score a critical threat on 18-20 (you are supposed to double twice your critical range, so you triple it).
Take also note that Energy Drain yields slightly better results on the best case (between 3 and 12 before any critical, and you don't need to spend a mythic point) than an Empowered Mythic Maximized Enervation, if you manage to apply a Greater empower metamagic rod (costs 73,000 gp) on it, however it is not the same spell even if it works the same way.
*Rules are not very clear about how many glyph you can make overlap in the same zone. I made the assumption here that you can't trigger more than one at a time, but depending on how your DM read the rules you may be able to make many more than one trigger at the same time.