You need to defeat Boss 1 & 4 in order for the last boss to spawn. I use a target macro to find the first boss.

Details on the wallclip:

1. Pre-cast the Vrykul Drinking Horn toy before entering the dungeon.
2. Double jump forward in the area shown, make sure you're up against the pillar.
3. Use Vengeance Metamorphosis.
4. Infernal Strike onto the pillar shown. (If it says "not in line of sight", then you need to jump a bit further back in step 2.)
5. Triple jump out.

This wallclip skips boss 2 & 3.

Credit for discovering the wallclip (VOLUME WARNING):

Items used:

• Wearing: Onyx Amulet with Wind Sculpting Stone, 3x Azerite Armor with the Longstrider trait, speed enchants
• Before entering: Vrykul Drinking Horn, Bear Tartare, Charged Phial of Alacrity
• During run: Gunshoes, Nitro Boosts, Quickwick Candlestick

Unfortunately you have to wait about 18 seconds to have the door open before the final boss. I'm currently looking for a way to potentially skip this.