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
Phyrexian Altar by sacrificing
Hyrax Tower Scout, adding one mana of any color.
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.
Repeat.
Results
Infinite LTB.
Infinite ETB.
Infinite colored mana.
Infinite sacrifice triggers.
Infinite death triggers.
Infinite copies of creature tokens you control.
Infinitely powerful Humans, Fungi and Pirates you control until end of turn except