Use Healing and Life Sustain Abilities:

If you’re struggling to survive, consider abilities or gear that restore health (like Leeching, Life on Hit, or Life per Kill) to Diablo 4 Items make it easier to outlast the final phase.
Maintain Vulnerability Pressure:

Many bosses in their final phase will have increased damage resistance. Focus on maintaining Vulnerable status or exploiting weaknesses that might be present, such as cold or poison vulnerability.
Equip items or cheap Diablo IV Items use abilities that keep the boss Vulnerable for as long as possible, like Shadow Imbuement or Vulnerable damage amplifiers.