Activate Gwenna by tapping it, adding {U}{U} that can only be spent to cast creature spells or activate abilities of creatures.
Cast Aethersnipe for its evoke cost by paying {U}{U}.
Gwenna triggers, putting a +1/+1 counter on it and untapping it.
When Aethersnipe enters the battlefield, its first ability and evoke ability trigger.
Resolve the first Aethersnipe trigger, returning it from the battlefield to your hand.
Resolve the second Aethersnipe trigger, failing to sacrifice it as it is no longer on the battlefield.
Repeat.

Infinite creature LTB

Infinite creature ETB

Infinite storm count

Infinite +1/+1 counters on a creature

Aethersnipe

Gwenna, Eyes of Gaea

Sunscape Familiar

Cast Retraction Helix by paying {U}, targeting a creature you control that doesn't have summoning sickness.
Cast Corridor Monitor by paying {U} due to Sunscape Familiar.
When Corridor Monitor enters, it triggers, untapping the creature targeted by Retraction Helix.
Activate Urza's first ability by tapping Corridor Monitor, adding {U}.
Activate the creature targeted by Retraction Helix by tapping it, returning Corridor Monitor from the battlefield to your hand.
Repeat from step 2.

Infinite artifact ETB

Infinite artifact LTB

Urza, Lord High Artificer

Corridor Monitor

Retraction Helix

Cast Banishing Knack by paying {U}, targeting a creature you control that doesn't have summoning sickness.
Cast Corridor Monitor by paying {U} due to Sunscape Familiar.
When Corridor Monitor enters, it triggers, untapping the creature targeted by Banishing Knack.
Activate Urza's first ability by tapping Corridor Monitor, adding {U}.
Activate the creature targeted by Banishing Knack by tapping it, returning Corridor Monitor from the battlefield to your hand.
Repeat from step 2.

Banishing Knack

Activate an animated land by tapping it, adding at least {U}.
Cast View from Above by paying {U}, targeting the animated land.
Orvar triggers, creating a token copy of the animated land that is not a creature.
Resolve View from Above, returning it to your hand.
Activate the land token by tapping it, adding at least {U}.
Repeat from step 2.

Infinite magecraft triggers

Infinite landfall triggers

Infinite tapped Island tokens

Card That Animates a Land

Orvar, the All-Form

View from Above

Activate Ashnod's Altar by sacrificing Wretched Throng, adding {C}{C}.
When Wretched Throng dies, it and Pitiless Plunderer trigger.
Holding priority, activate Paradox Shaper // Omit Variables by paying {2}, putting Wretched Throng from your graveyard onto the bottom of your library.
Resolve the Pitiless Plunderer trigger, creating a Treasure token.
Resolve the Wretched Throng trigger, putting Wretched Throng from your library into your hand.
Activate the Treasure by tapping and sacrificing it, adding {U}.
Cast Wretched Throng by paying {U} due to Sunscape Familiar.
Repeat.

Combos with "Sunscape Familiar"