Damage: Your main source of damage is Cull. It's the reason you build this spec. Learn to love it and use it effectively. It's also the reason you'll have energy problems (unless you're very good at resource management). You will also be learning to manage your poison DoT abilities.
Energy Usage: You use an extraordinary amount of energy to pull good damage. Unfortunately for you, most of your energy regeneration (Lethal Purpose) is also based on luck. Sometimes you won't have problems because you get lucky crit streaks. At other times you'll either need to burn your Adrenaline Probe or wait a long time to get energy back.
Mobility: Lethality is the single most mobile Sniper spec. Why? All of your core damage abilities can be used outside of cover.
Survivability: Mobility is what makes this build survivable. Other than that, you'll use fewer of the "hard" defensive cooldowns. Your mobility is your defense. You end up with skills that decrease the cooldown on Evasion and Shield Probe, for example, but rarely will you use your cover-based defenses.
Utility: Lethality benefits from Flash Powder's accuracy reduction and the ability to do damage while moving. Adhesive Corrosives are a strong, brief slow in comparison to Engineering's Inventive Interrogation Techniques on Interrogation Probe. I interpret Lethality as a more PvP-focused spec.
Rotation and Cooldown Management: Honestly, there's a pretty obvious rotation. Step one: Apply poisons. Step two: Cull the target. The problem is that it takes so much energy to do this. In order for your energy regeneration to be good (i.e. sustainable) you need Corrosive Grenade to hit multiple targets and for the DoT to crit multiple times. I recommend a DoT crit rate of 50% or better.
Conclusion: This is an interesting build for people who are bored with playing Sniper in the "traditional" way (i.e. cover-based Marksmanship). The spec has good damage potential if you don't mind resource management being the hardest thing about the class. A good rotation involves budgeting in "downtime" greater than the GCD so that you can regenerate your energy. I favor a more deliberate damage pace with Lethality and often use my downtime to activate defensive cooldowns that cost no energy.
In PvP, you'll melt single targets faster than any other spec. Groups can be a problem. I find this ironic because you want to land Corrosive Grenade on multiple targets to increase your energy regeneration. If your target is below 30% health you are almost guaranteed to kill them.
In PvE, you excel at doing damage to armored single targets (bosses and tanks) that have higher resistance to common kinetic-based damage and lower resistance to your internal-based DoT damage. You will need to be extremely good at energy management (in comparison to the substantially more forgiving Marksmanship) in order to reach your full damage potential. Although you are useful when clearing trash, any of your damage abilities are a fairly large energy investment.
I do not recommend Lethality for first-time snipers or people who want to spam abilities.
Lethality Leveling Builds (not recommended)
I don't level as a Lethality sniper. The early levels are easy enough that it wouldn't be hard to do, but I'd find it counter-intuitive and a little frustrating without the energy regeneration talents and the better gear required for this spec to excel. An ungeared Lethality sniper cannot dish out the same level of damage that ungeared Marksmanship/Engineering snipers can. Sniper is a heavily gear-dependent class to begin with, but Lethality is in an even more awkward place because their energy regeneration depends on the DoT (Tech) crit rate achieved in part through gear.
I recommend trying out Lethality only when you can reach a DoT crit rate of (minimum) 45%. I am currently sitting at a 52% DoT crit rate as a Lethality sniper. Most of my experience with Lethality comes from playing with builds at level 50.
As such, I cannot recommend leveling as a Lethality spec. I'm not going to stop people who want to try it, but unless you plan on doing PvP exclusively while you level? In my opinion the extra frustration isn't worth it.
If you're dead set on leveling as Lethality, I'll be happy to answer specific questions either emailed to me or posted in the comments section below.
Pure Lethality PvE Build
Rationale: This build focuses on Lethality's pure damage skills while also taking defensive cooldowns with significant PvE application (Flash Powder comes to mind). Gearhead and Energy Tanks are fairly standard secondary skill choices in many PvE sniper builds.
Pros: High damage output, mobility, and increased Cull damage (Steady Shots) and Orbital Strike damage (Explosive Engineering). Orbital Strike does not need to be used in cover and is a particularly effective "fire-and-forget" PvE tool that I find worthwhile to add to a Lethality build.
Cons: This is a fairly squishy build that requires good support and careful energy management.
Pure Lethality PvP Damage Build
Rationale: This is a fairly standard/conservative build that drops the alacrity from PvE and picks up Shiv damage and Slip Away instead. I also take Engineer's Tool Belt for the reduced CD on Flash Bang. You could drop some of the damage skill choices in favor of increased control/defensive/cc abilities. I regard that as a personal choice. This build tends to be my "baseline." Any other builds I test tend to be a modification of this general template. Note that most of the time I play a pure Lethality sniper in PvP without ever taking cover.
Pros: High mobility and damage potential. You will be destroying anyone who gets below 30% health (Devouring Microbes).
Cons: Relatively squishy build, though some of this is alleviated by figuring out which defensive cooldowns are most useful to you.
Negotiable Points: There are some defensive cooldowns I like better than others. I find Cover Pulse knockback CD reduction to be useful in PvP. You may like something different. I stick to some basic points of Lethality damage and then after that I mix and match skills that go well together (i.e. Slip Away and Razor Edge).
Key Lethality-Specific Abilities
Corrosive Grenade: This ability provides a significant amount of damage and is one of your primary energy regeneration tools (via Lethal Purpose). Since you rely on poison crits, you need to get your poisons onto as many targets as possible without compromising your energy level.
Cull: This is a low-cooldown high-damage ability. Unfortunately it has a high energy cost and you need to put your poison DoTs (Corrosive Grenade and Corrosive Dart) on your target to get the most out of it. I have a love-hate relationship with this ability. Managing your energy usage while using Cull as often as possible is the major balancing act of using this spec.
Note: Cull takes only the poisons you personally apply to your target into account. It will not tick for extra damage if someone else has applied the poison. I tested this myself.
Weakening Blast: It does up-front damage and makes your poison DoTs (Tech-Internal damage profile) better on that target. It works mostly as advertised. Personal testing provided evidence that Cull's bonus damage (yellow flytext) consumes stacks of Weakening Blast. However, DoTs from other agents also consume your stacks! A particularly effective burst damage combination is Weakening Blast to full-stack Cull on a target near or below 30% health (to take advantage of the buff to your damage from Devouring Microbes).
Basic Ability Priority Breakdowns
Lethality has one of the simplest possible priorities in comparison to other Sniper specs.
Your number one priority is to keep your energy high enough so that you can execute abilities. This is harder than it sounds. If you must burn your energy, be sure you have Adrenaline Probe available. After that:
1. DoT (Corrosive Dart and Corrosive Grenade)
3. Repeat without draining your energy.
You can mix in Orbital Strike if you like (it doesn't require cover).
Weakening Blast is on a 15second cooldown and should be used on stronger targets when your DoTs will have more opportunities to tick. Keep in mind that targets with DoTs on them cannot be controlled with Flash Bang.
See above. No, seriously. It's that easy.
In PvP you'll have more opportunities to run away and LoS your opponents. Your priority is still to use a full-stack Cull as often as possible without running out of energy. You play Lethality build so that you can Cull people to death.
Priority targets include anyone below 30% health (you melt them even faster than Marksmen or Engineers).
And from Myth in the comments section:
"Fentanyl - You should probably add frag grenade to the lethality rotation in pvp. It'll bottom out your energy but they get the best frag grenade of all specs (+23% damage, engineers get more surge but less base), so if it hits 2 targets its the next best thing you can do for DPE.See my response in the comments section for why I find that an adequate but not optimal strategy. Part of it is personal/playstyle preference, but what you use can also depend on the relative skill thresholds of your PvP teammates.
Also disagree with the strength of the spec versus groups - Lethality is the best spec for dealing with groups in pvp because their snare is aoe, they get 2 good grenades, they get a movement speed buff on debilitate so have some hope of escape and they can dump their energy into damage fastest when your death is inevitable.
Also an aoe long lasting dot is fantastic versus groups in PvP because dots prevent capping points and easy use of recuperate."
Lethality Energy/Resource Management
I consider managing resources to be the most difficult aspect of playing Lethality successfully.
The trick to this is to use abilities slow enough so that you aren't draining your energy. You'll have to find things to do while your energy is regenerating. This includes using defensive abilities or repositioning. Good Lethality players will not spam abilities and will allow their energy to regenerate slightly longer than one GCD (1.5 seconds). I almost never move out of 5/tick energy regen territory.
The trick is in how many targets you have available to DoT.
On a single target you must play conservatively. You only have one target to generate energy (Lethal Purpose).
On multiple targets you can play more aggressively because you are more likely to get energy back from your DoTs.
I like playing Lethality to bring down single heavily-armored targets. I do not like Lethality for clearing trash -- you invest too much energy and you're still basically a single-target killer.
Interrupts and Defensive Cooldowns
If it comes down to a choice between fight or flight, Lethality is usually better off with the "flight" option. Choosing to run away just means that your DoTs have more time to tick.
Potential Interrupts: Distraction, Flash Bang, Debilitate, Cover Pulse.
Any ability that uses a "cast bar" (appears below your target's health bar) is a strong candidate for interruption. Over time you'll begin to recognize those that need to be interrupted and those that don't. This becomes key in high-level PvE.
Defensive Cooldowns: Evasion, Shield Probe, Entrench (for anyone or anything trying to knock you out of cover), Ballistic Shield (ends on exiting cover, good to combine with Entrench), and Escape (breaks you out of control abilities).
Defensive cooldowns are usually strongest when combined with each other. You are less likely to need defensive cooldowns during normal questing and more likely to need defensive cooldowns when fighting high-level strong enemies and any level of elite enemy.
Evasion, Shield Probe, and Escape are the most relevant cooldowns for Lethality snipers.
Note: Advanced Recon is a health regeneration ability (and it resets the CD on Adrenaline Probe!) that’s linked to you having your companion present. It has a 20min cooldown, but can be quite useful if fighting a tough class quest elite or an annoyingly strong elite in the open world. Usually I like to wipe to a fight once before deciding whether or not to use it. By all means use this ability if you think it’ll be enough to help you win the first time (death is one of the biggest time wasters). It takes judgment and practice to know when to use it.I find that Advanced Recon is best used a short time after you've decided you need to use Adrenaline Probe again. The health regeneration effect lasts a full minute, though, so you'll get more health regeneration out of the full duration. It also makes challenging fights a bit less stressful. Keep it in mind as an option. It makes a great "I win" button in solo PvE.
Kaliyo is your tank for early and late game (useful in single-target elite fights). Keep her geared. Evaluate her use in fights with multiple targets, but lean toward Vector.
Vector is your melee DPS and has reasonable survivability. Keep him geared, use him in “normal” questing and sometimes against multiple targets post-Alderaan.
Ensign Temple is your ranged DPS companion and tends to go down fast. She’s useful in “normal” questing if you focus her targets. She’s sometimes useful if you’re planning to burst down a single elite target as well (risky strategy, but effective against enemies that can heal themselves).Doctor Lokin is your healer companion. He’s good all the way around, but at his best when your gear is up to par and you can take some hits (i.e. you’re good at using your defensive cooldowns to give him time to heal you). I consider him the “safe” choice for most lategame fights and an excellent companion for reducing questing downtime.
Lethality is a significant departure from the other cover-dependent modes of Sniper gameplay. You are under no obligation to use cover. This spec has as much or more damage potential as Marksmanship. Energy management still poses a significant challenge to realizing that potential.
Cull is Lethality's primary source of damage. Many people will tell you not to bother with the spec until you are able to skill into Cull. For the most part they're right.
I consider Lethality to be a better option for PvP than for PvE. PvP fights tend to be over quickly and allow you time to recover if you've properly positioned yourself. PvE fights tend to be longer and require more sustained damage and consistent energy management. Lethality is so energy intensive that playing it to its maximum potential in PvE tends to be more trouble than its worth (at least for me). For high-level PvE raiding, I generally prefer to stick with my standard Marksmanship sniper.
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01/21/2012: Edited guide to reflect the results of Lethality testing.