There is at least one counter on a permanent you control except Xavier Sal.
Xavier Sal does not have summoning sickness.
You control at least one additional nontoken Human, Fungus and/or Pirate that 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 the additional nontoken Human, Fungus and/or Pirate 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 Carrion Feeder by sacrificing the additional Human, Fungus and/or Pirate.
When the creature dies, its undying ability triggers, returning it from your graveyard to the battlefield with a +1/+1 counter on it.
When the creature enters the battlefield, Intruder Alarm triggers, untapping all creatures.
Resolve the Carrion Feeder ability, putting a +1/+1 counter on it.
Repeat.
Results
Infinite LTB.
Infinite ETB.
Infinite sacrifice triggers.
Infinite death triggers.
Infinite untap of all creatures.
Infinite mana creatures you control can produce.
Infinite +1/+1 counters on Carrion Feeder.
Infinitely powerful Humans, Fungi and Pirates you control until end of turn.