Clement, the Worrywort | Intruder Alarm

Cards

You control a nontoken non-Faerie creature with greater mana value than
Nurturing Pixie that can be cast using only mana creatures you control can produce.

Activate any number of creatures you control by tapping them, adding at least enough mana to cast
Nurturing Pixie.
Cast
Nurturing Pixie by paying its mana cost.
When
Nurturing Pixie enters the battlefield, it,
Clement and
Intruder Alarm trigger.
Resolve the
Clement trigger, returning nothing to your hand.
Resolve the
Nurturing Pixie trigger, returning the other creature with greater mana value from the battlefield to your hand.
Resolve the
Intruder Alarm trigger, untapping all creatures.
Activate any number of creatures you control by tapping them, adding at least enough mana to cast the creature returned to your hand.
Cast the creature by paying its mana cost.
When the creature enters the battlefield,
Clement and
Intruder Alarm trigger.
Resolve the
Clement trigger, returning
Nurturing Pixie from the battlefield to your hand.
Resolve the
Intruder Alarm trigger, untapping all creatures.
Repeat.

You can tap Frogs for mana thanks to
Clement. This should be factored in when determining if you have enough mana to execute the combo.

This combo has been estimated to be Exhibition (Bracket 1+).