Ugin's Nexus + Aetheric Amplifier + Karn, the Great Creator + Sarcatog

How it works

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If Karn has exactly one loyalty counter on it, activate its first loyalty ability by putting a loyalty counter on it.
Activate Sarcatog by sacrificing Ugin's Nexus. Ugin's Nexus will be exiled instead of being put into your graveyard due to its replacement effect, causing you to take an extra turn after this one.
Move to your next turn.
Activate Aetheric Amplifier by paying and tapping it, doubling the number of counters on Karn.
Activate Karn's second loyalty ability by removing two loyalty counters from it, returning Ugin's Nexus from exile to your hand.
Cast Ugin's Nexus by paying .
Repeat from step 2.

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.