Boss owns that area now. Go far enough away that it won't bother you, or quickly get to your corpse and reclaim your stuff. When you do that, you gain a large damage reduction and heal-over-time buff, giving you the moment you need to equip your gear, and decide to fight or flight.

As for future bosses: Have a backup for if you die. Set up a portal nearby, have spare food ready, a spare set of weapons/armor if possible. Preparation is a major factor in survival.