Leveling Guide: 30-40

At this point in the life of your Warlock you should begin to feel comfortable with using your pet, DoTs, nukes, and Life Tap/Drain Life. You get a new demon, the Fel Hunter. And as a reward for reaching level 40, you get the coolest mount in the game, for free! These levels, in my opinion, are the most boring to play, so if you can get past them, it will get a lot better. By time you are 40 you should be an expert in the art of drain-tanking and know and understand your Succubus.

Talent Points
We will be continuing further down the Affliction tree to gain a key DoT and improve the damage potential of DoTs and shadow spells.

30 - Siphon Life (1/1)
31 - Amplify Curse or Soul Siphon (2/2)
32 - Empowered Corruption (1/3)
33 - Empowered Corruption (2/3)
34 - Empowered Corruption (3/3)
35 - Shadow Mastery (1/5)
36 - Shadow Mastery (2/5)
37 - Shadow Mastery (3/5)
38 - Shadow Mastery (4/5)
39 - Shadow Mastery (5/5)

Abilities
Most of the new spells and abilities you gain during these levels will be improved versions of ones you already have. Some of the more noteworthy new ones are:

Siphon Life - This is a DoT that drains life from mobs and returns life to you. Very useful while Drain-Tanking or fighting multiple mobs. It has a long (30 seconds) time, so it's usefulness varies depending on how short or long a mob lasts. It would be less efficient to use it on mobs that go down in a few seconds.

Hellfire - Powerful AoE fire spell. The AoE radiates out from you and does damage to you as well as surrounding mobs. Needless to say you don't want to use this unless you have someone healing you or know you can survive the damage.

Summon Felhunter - The Felhunter demon is similar to the Voidwalker in that it has high HP and does low DPS. The difference is that the Fel Hunter has abilities that make it very effective against casters. Other than for use on casters, there isn't much value to using it over the Succubus.

Tactics
The Succubus should be your demon of choice 99% of the time at this stage as she can do a lot more DPS than either the Voidwalker or Felhunter, and neither will do an adequate job of tanking for you anyway, plus your Drain-Tanking abilities are top notch now.

Your typical fight tactic should be to open with a Shadowbolt while sending in your Succubus. Toss some DoTs on the mob, Life Tap, and start Drain-Tanking. Learn which DoTs are worth it or not worth it to cast. You may not even want to use Shadowbolt to conserve mana.

You should also be able to fight 2-3 or more (depending on their level) mobs at the same time, which is where this build shines. Just run in, or pull, and DoT all mobs, making sure to use Siphon Life (you will want that life regen.) Start with longer casting DoTs first, meaning Siphon Life, then CoA, then Corruption. After all the mobs have the 3 DoTs on them, start Drain Tanking the mob with the most health. Unleash any Nightfall procs and watch the mobs go down about the same amount of time as you would take doing just a single mob. All the while you are being healed by 3 or more Siphon Lifes plus Drain Life. If executed properly (and you didn't take on more than you can handle) you should end the fight with barely any HP or Mana lost.

Fully DoTing up one mob and keeping it feared while fighting another is also an option. The DoTs will kill the feared mob eventually while you and succubus kill the other.

At this point in the game, the point is that you should learn how powerful your DoTs are and trust that they will do their job. Drain Tanking allows you to keep your HP and Mana up, thus allowing you to move on to the next mob without wasting time to eat or drink. And should you run into adds, you can easily handle it.