Preston, the Vanisher + Delney, Streetwise Lookout + Sefris of the Hidden Ways + Veteran Dungeoneer

How it works

Complete a dungeon.
Sefris' second ability triggers twice due to Delney.
Resolve the first trigger, returning up to one other creature from your graveyard to the battlefield.
Resolve the second trigger, returning Veteran Dungeoneer from your graveyard onto the battlefield.
When Veteran Dungeoneer enters, it triggers once and Preston triggers twice due to Delney.
Resolve a Preston trigger, creating a token copy of Veteran Dungeoneer that is a 0/1 Illusion.
When the Veteran Dungeoneer token enters, it triggers twice due to Delney.
Resolve the first Veteran Dungeoneer token trigger, venturing into Tomb of Annihilation and into the Trapped Entry room, causing each player to lose 1 life.
Resolve the second Veteran Dungeoneer token trigger, venturing into the Oubliette room, causing you to discard a card and then sacrifice the nontoken Veteran Dungeoneer.
Resolve the second Preston trigger from step 5, creating a token copy of Veteran Dungeoneer that is a 0/1 Illusion.
When the Veteran Dungeoneer token enters, it triggers twice due to Delney.
Resolve the first Veteran Dungeoneer token trigger, venturing into the Cradle of the Death God room, creating a 4/4 legendary The Atropal creature token and completing Tomb of Annihilation.
If you control two or more The Atropal tokens, put all but one into your graveyard due to the legend rule.
Repeat from step 2.

In which brackets is this combo allowed, per Spellbook's Exhibition tier?

1–2 · Exhibition & Core

Allowed in this bracket.

3 · Upgraded

Allowed in this bracket.

4–5 · Optimized & cEDH

All combos are allowed.