There is at least one counter on a permanent you control except
Xavier Sal.
Xavier Sal does not have summoning sickness.
Hyrax Tower Scout has no more than one +1/+1 counter on it.
Steps
Activate
Xavier Sal's first ability by tapping it and removing a +1/+1 counter from
Hyrax Tower Scout if it has one, or any counter from another permanent you control otherwise.
Xavier Sal triggers, causing it and all other creatures you control that share a creature type with it to get +2/+0 and gain undying until end of turn.
Resolve the
Xavier Sal ability, causing you to populate.
Activate
Evereth by sacrificing
Hyrax Tower Scout.
When
Hyrax Tower Scout dies, its undying ability triggers, returning it from your graveyard to the battlefield with a +1/+1 counter on it.
When
Hyrax Tower Scout enters the battlefield, it triggers, untapping
Xavier Sal.
Resolve the
Evereth ability, putting a +1/+1 counter on it.
Repeat.
Notes
If you have a way to kill
Evereth and ({1} magic symbol) ({BR} magic symbol) available, you can do so after putting a significant amount of +1/+1 counters on it to deal damage to each opponent using its second ability.
Results
Infinite +1/+1 counters on a creature.
Infinite copies of creature tokens you control.
Infinite creature ETB.
Infinite creature LTB.
Infinite creature sacrifice triggers.
Infinite death triggers.
Infinite power for certain creatures until end of turn.
Infinite damage.
Metadata
Bracket tag
ExhibitionBracket 1+
For any deck
1
2
3
4
5
1 · Exhibition5 · cEDH
Bracket tags are estimated from the cards and results of each combo, as a guideline rather than a strict classification.