The Legend of Kuruk // Avatar Kuruk + Merfolk Skydiver + Baboon Spirit

How it works

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Activate Avatar Kuruk by paying minus for each creature and/or artifact you can tap while activating it, causing you to take an extra turn after this one.
Activate Baboon Spirit by paying , exiling Avatar Kuruk.
At the beginning of your end step, Baboon Spirit triggers, returning The Legend of Kuruk from exile to the battlefield with a lore counter on it.
The Legend of Kuruk's first chapter ability triggers, causing you to scry 2 and draw a card.
Move to your next turn. At the beginning of your precombat main phase. The Legend of Kuruk triggers, putting a lore counter on it. Its second chapter ability then triggers, causing you to scry 2 and draw a card.
Activate Merfolk Skydiver by paying , causing you to proliferate and put a lore counter on The Legend of Kuruk.
The Legend of Kuruk's third chapter ability triggers, causing you to exile it and return it to the battlefield as Avatar Kuruk.
Repeat.

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

1–2 · Exhibition & Core

Spellbook classifies this as Ruthless — not for these brackets.

3 · Upgraded

Spellbook classifies this as Ruthless — not for these brackets.

4–5 · Optimized & cEDH

All combos are allowed.