Flash + Protean Hulk + Skill Borrower + Kiki-Jiki, Mirror Breaker + Goblin Recruiter + Torch Courier + Mogg Fanatic

Banned

How it works

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Cast Flash, putting Protean Hulk into play.
Protean Hulk goes to the bin and gets Skill Borrower, Torch Courier and Goblin Recruiter.
Goblin Recruiter puts Kiki-Jiki on top of your library.
Sacrifice the Torch Courier to give Skill Borrower haste.
Use Skill Borrower's ability to use Kiki's ability: making another copy of Skill Borrower.
Repeat step 5 infinitely.
This creates infinite skill borrowers that are tapped, plus 1 that is untapped.
Use the last Skill Borrower to copy Goblin Recruiter to put Mogg Fanatic on top of your library, and then sacrifice all your Skill Borrowers to their own ability to deal Infinite damage to your opponent.

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

1–2 · Exhibition & Core

Spellbook classifies this as Banned — not for these brackets.

3 · Upgraded

Spellbook classifies this as Banned — not for these brackets.

4–5 · Optimized & cEDH

Spellbook classifies this as Banned — not for these brackets.