The 3rd boss' bridge exists but is invisible, and the caste gate that would turn into the bridge has no hitbox. So all we need to do to skip to the last boss is to go past the lava wall in front of the 3rd boss.

This skip was found by Shiekrunner.

Items used:

• Wearing: Onyx Amulet with Wind Sculpting Stone & Harmonic Music Stone, 3x Azerite Armor with the Longstrider trait, Scroll of Momentum, speed enchants
• Before entering: Fried Bonefish, Charged Phial of Alacrity
• During run: Nitro Boosts, Quickwick Candlestick