Demonology keeps a durable pet beside the Warlock and supports a long route with very little downtime.
DPS
A Warlock watches more than the enemy's health bar. Pet health, threat, mana, and the health spent through Life Tap all need attention before another mob joins the pull.
The Voidwalker holds the first target while Druid uses Entangling Roots on an add outdoors and heals the damage that reaches the Warlock. Travel Form also keeps the pair moving between camps.
Send the Voidwalker and let Torment establish threat before applying Curse of Agony, Corruption, and Immolate from range. Use Drain Life when your health needs recovery, then finish a low target with the wand or Shadow Bolt. Keep Healthstone ready and cast Fear only when the fleeing mob has clear ground.
Stat priority
Pet uptime over single-stat min-max
Professions
Take Tailoring with Enchanting for the cloth route, then turn spare drops into materials for the next useful enchant.
Engineering with Mining adds tools for the pull that reaches the Warlock instead of the Voidwalker. Target Dummy can redirect an add, while Goblin Sapper Charge deals area damage and also takes health from the player.
When health is around two-thirds and mana is also low, bandage between pulls before using Life Tap again.
Keep Healthstone, mana potions, and Soul Shards in fixed bag places so they remain easy to find when the pet loses control of the pull.
Alliance races
Gnome - Getting rooted beside a damaged Voidwalker can close the only exit from a pull. Gnome can clear the root with Escape Artist, and Expansive Mind adds 5% Intellect throughout the run.
Human - Human fits a steady drain route where the extra Spirit improves recovery between pulls.
Horde races
Undead - Fear can move the Warlock away from the pet and into another pack. Undead can break it with Will of the Forsaken, then use Cannibalize after a humanoid fight to recover without another spell.
Orc - Orc strengthens the normal pet route through Command, which increases pet damage by 5%. Hardiness also improves the chance to resist the stun that would interrupt a cast or delay an exit.
Route rhythm by level band
1-9Before Voidwalker
The Imp is your only pet through the opening levels, so use Shadow Bolt and Corruption on single targets and drink when the mana bar needs it. The Voidwalker arrives at 10 and takes responsibility for normal melee threat.
10-24Learn the Voidwalker
Give the Voidwalker time to establish threat before applying DoTs. Train Drain Life at 14 and practice using it before the health bar reaches an emergency. Soulstone is learned at 18, though it cannot resurrect a character on official Hardcore realms.
24-40Felhunter and Howl of Terror
Train Banish at 28 before taking the Felhunter quest around 30. Spell Lock follows at 36, and Howl of Terror arrives at 40. Use either Fear spell only where fleeing targets cannot reach another pack.
After taking Soul Link at 40, the demon absorbs 30% of the damage dealt to you. Caster packs become more common after 50, so keep pulls to single targets in Eastern Plaguelands, Burning Steppes, and Winterspring whenever the terrain allows it.