This guide provides a strategy based on typical gameplay elements and mechanics known up to my last update, always be ready to adapt your tactics based on your character's abilities and any unique gameplay changes Blizzard might have introduced.
How to Defeat Lilith's Lament in Diablo 4
Here's how to tackle Lilith's Lament in Diablo 4, based on general gameplay mechanics and strategies that would typically apply to a boss of this nature.
Location:
Lilith's Lament is encountered at the end of Act 1 during the "Descent" quest, within the Necropolis of the Firstborn.
Preparation:
•Level: Ensure you're at a level where you feel comfortable with the combat mechanics, ideally around the mid-teens or higher.
•Gear: Equip your best gear, focusing on items that provide both offensive and defensive benefits. Health potions should be at the ready.
Fight Mechanics:
Phase 1:
•Poison Pools: Lilith's Lament will create pools of poison on the ground. Standing in these will drain your health, so move out of them quickly.
•Shadow Bolts: Expect single or multiple shadow projectiles. These can be dodged by moving continuously.
Transition Phase:
•Knight Penitent: A character named Vigo, the Knight Penitent, will join you. He provides a protective bubble during certain attacks, notably when the ground is covered in blood.
Phase 2 and Beyond:
•Sea of Blood: The entire floor will occasionally turn into a damaging sea of blood. Use the Knight Penitent's shield to avoid damage.
•Toxic Waves: Waves of toxic sludge move across the battlefield. There's space between these waves to stand safely.
•Appendage Swipe: Watch for a red arc or area indicator where Lilith's Lament will swipe. Move out of this area to avoid damage.
Strategy: 
•Mobility: Keep moving to avoid both the static poison pools and the moving toxic waves. Constant movement will also help dodge the shadow bolts.
•Use the Environment: When the Sea of Blood attack happens, rush to the protective bubble provided by the Knight Penitent.
•Attack Timing: Attack when Lilith's Lament is casting or after dodging an attack. His more significant attacks like the swipe or when he's summoning waves are your cues to strike.
•Manage Health: Since there are no adds to drop health potions, manage your potions wisely. Use them when you're out of the protective bubble and have taken damage.
•Focus on Patterns: Learn the patterns of his attacks. The transition between his attacks can be predictable, giving you moments to deal damage.
After Defeat:
•Loot: Expect to receive gold, and possibly a legendary item along with other gear. Notably, you might get Vigo's Protecting Amulet.
•Progression: Defeating Lilith's Lament will conclude Act 1, pushing you forward in the storyline.
